Is it a PTB move to ban a user if their only activity in a community is downvoting posts?

Ban them.
Lemmy is so young (and feeble) that users like those are an actual threat to your community and the larger network by driving away those who actually contribute to the community. In 2019, TrueBirch from Reddit analysed the data and concluded that only 1.9% of users actually comment or post while 98.1% just lurks. When your community is has a thousand or so users, it's entirely reasonable to protect those ~20 users who are creating content for the rest. In fact, the majority of the rest likely don't upvote things either.
- I've heard this described as "the 1% Rule", which more or less goes like: In online communities, 1% of the users generate 90% of the content, 9% of the users create 10% of the content by reacting to, modifying or generally interacting with that 1%, and the other 90% of people are lurkers. This fits quite well with what I've seen on online communities myself for decades. So, if you alienate that 1%, your community will eventually either disappear or become a hollow reflection of what it used to be.
  
  Absolutely, and it's already pretty hard to bootstrap a community organically† to so you should not hesitate to do what's necessary to keep it healthy as small communities cannot moderate themselves easily.
  † From How Reddit Got Huge: Tons of Fake Accounts:
  Well, according to Reddit cofounder Steve Huffman, in the early days the Reddit crew just faked it ‘til they made it. In the above video for Udacity, an online source for education and lectures, Huffman describes how the first Redditors populated the site’s content with tons of fake accounts.
- Came in with the opposite view, but this convinced me and changed my mind.
  
  Glad to hear! As I said, Lemmy is still so young that it makes perfect sense. Even much more mature and much larger communities take similar measures:
  Hacker News users cannot downvote anything until their karma is > 500 and even then only comments (not submissions) source.
  Some large Reddit communities such as r/politics hide downvote buttons altogether or for non-subscribed users. They even ran a study back in 2018: Does Hiding Downvotes Improve Behavior in r/politics?
  Community cultures vary widely, but in the case of r/politics, hiding downvotes does not appear to have had any of the substantial benefits or disastrous outcomes that people expected.
  I'm not saying that downvotes are bad, but that people abusing downvote mechanism are bad and that it's okay to ban such users while bootstrapping a community.
- I'd argue that lemmy is so young, and small, that communication first is a viable, and better, way to start.

I feel this could go several ways.
Troll who just wants to make people feel bad. Ban them.
Person who legitimately thinks content is low-quality, but likes the topic in general (perhaps no other Lemmy community exists for it?) and wants to see better posts. Leave it alone.
Drive-by voter checking in from Local or All (sorting by New makes even small communities visible, so "we're not big" isn't immune), maybe they legit think it's low effort, or hostile and toxic. Or they just see Thing They Don't Like and downvote instead of using the system properly—not quite the same as a troll purposely, maliciously ruining things but in my opinion not a great practice at all, but also not quite a bannable offense, unless they actively subscribe to a community full of things they do not like just to shit on it. And I am guessing right now intent is pretty hard to prove.
Because I can think of reasonable reasons to do this I would err on the side of not punishing a potential innocent. I'd wait at least until you see several extremely high-quality posts that are also inoffensive get downvoted. But I also get that downvoting everything in a community can be destructive. A big community can just shake it off, the upvotes will eventually outweigh the few downvotes. A small one will look like it has crap content if there is ~3 up/3 down to every post.
Also, check patterns outside your community too. Explicit stated intent to ruin community in a comment? Goodbye. Upvotes things outside the community? More chance to be a normal user who's probably got an innocent reason to downvote.

I’m a serial voter (up and down), I have an opinion about practically everything. If anyone as a result gets a lot of down votes, they should perhaps create more content that I like? Just a suggestion.
- That’s not bad per se but is your vote based on whether you agree with something or whether it contributes something of value to the community?
  Many people use downvote as a „dislike” button which defeats the purpose of news-based communities where it’s the newsworthiness that matters. Of course I don’t like that Russia bombed children but I will upvote it so more people see it, for example.
  
  I see that the same way.
- Have you ever downvoted all of the 10 most recent posts in a community?
  
  By coincidence this could have happened - eg if ten posts were off topic.
- I have an opinion about practically everything
  Where is the /s?
  they should perhaps create more content that I like?
  Your account is less than 15 days old. You have 57 comments and one single post about politics. Not exactly the type of discussion that is interesting or valuable to the network.
  Perhaps you should lead by example and post more of the stuff that you like? It's very easy to criticize, not so easy to actually show up and do the stuff that is needed.
  
  Didn't even notice, thank you for pointing this out

My initial reaction when I read your posts was 100% support for banning them. They'd be better off blocking the community so it never shows up in their feed. But in the hours since, a counter argument occurred to me that I'd like to present.
Lemmy is, as many have said, still a relatively young and small platform. The range of content on it is limited not just by what subjects have communities, but what the individuals posting in those communities happen to be interested enough in to share.
A user might not want to block a community because they do like the subject of that community and want to read about it. But maybe only one user is actively posting content, and the subset of interest in the subject for user A just doesn't match the subset of user B. Naturally, they end up downvoting all the content at present. Where in a hypothetical that there were hundreds of active users posting in that community, the user would only downvoted a small fraction.
As a hypothetical, imagine I create a !classicalmusic community. I'm a big fan of classical music, especially Beethoven and the Romantics. I post a heap of discussion about Romantic theory, recordings of Beethoven Symphonies, Rachmaninoff Sonatas, etc. Because it's a small community, I'm the only one posting.
Then you come along, a huge Bach fan. You don't mind some Classical era stuff like Mozart and Haydn, but you can't stand the Romantic era. You downvote everything I post.
In my opinion, unless you want to get even more into the weeds and enforce the idea of "downvotes are only for off-topic and spam content, not for dislike" (which, I agree in theory is how the best users treat it, but let's be honest…it doesn't happen in practice), I don't think I should ban you.
Maybe I could send off a DM asking you to explain your downvotes, and I would ban you if you came back and said "I don't want to see classical music in my feed" (along with a recommendation that you use the block feature). And I'd try to encourage you to participate more in submitting the stuff you do want to see. But an attempt to figure things out some other way would be better than a ban, in that case.
- Then you come along, a huge Bach fan. You don't mind some Classical era stuff like Mozart and Haydn, but you can't stand the Romantic era. You downvote everything I post.
  Something about this would seem kind of selfish to me. Just because I don't like the Romantic era doesn't mean I should try to bend the entire !classicalmusic community to my will. My efforts would be better spent posting to or creating !baroquemusic, !romantic_era, etc.
  As it happens, I happen to like both the baroque and romantic eras :)
  I'll leave this here for anyone else who might be interested: !classical@lemmy.studio
  
  Right, but this comes down to that same discussion I alluded to but largely wanted to avoid (due to irrelevance) about whether the downvote is for disliking content, or whether it should only ever be for off-topic and spam. You're just never going to get a situation where people stop using it for content they dislike.
- You downvote everything I post.
  Seems like downvoting everything you post instead of posting some Bach content is quite counterproductive.
  
  Is it? It's already been established elsewhere in this thread that there's a pyramid of users. The majority lurk. The cast majority of what's left only ever vote. The majority of the rest only comment. And only a small fraction actually create posts. We're not going to change that just by hoping.
  But you're right, it would be better if they posted. And that's why I suggested DMing them to initiate a conversation. I'd ask them if they could post more of their own stuff.

You need to know why.
I went through days on Reddit where I would not find something posted that I wanted to upvote and found posts that, while maybe good for another community, were not fitting my definition of the intended topic. As a non-mod, you can't directly shape the content, but you can vote.
Like you said, they also might not understand that downvoting things from a particular community will not make it show up less.
Just ask them. If they're a jerk, you can still ban them. If they are ignorant but cool, you can help them be a better netizen.

I've encountered a few of those users, and personally I ban them.
My criteria:
They must be downvoting a majority of posts, where it's clear there is no real discrimination on quality (as then some posted close together would be spared)
I notice it happening across multiple communities/instances
I see the same name again and again, day after day
I never see that user upvote anything anywhere
I know there was one user who was mass downvoting to, according to them, mark posts as read.
I thought that was fairly ridulous, as why not just mass upvote instead to achieve the same effect? Wasn't sure I fully bought that, as they were dismissive when someone asked them to stop.
Lemmy is still small, and it seems unwise to allow mass downvoting to potentially discourage or limit the reach of people putting in the effort to help this place grow.
I've never had a downvoter message me to ask why they were banned or dispute it, so I figure they didn't even notice that they no longer saw the stuff that caused them to downvote in the first place.
- I doubt anyone would ever object to banning for the behaviour you described here. But unless I'm way off base, I don't think that's what OP is talking about.
  What you're talking about is basically inauthentic behaviour. Maybe it's a bot, maybe it's a real person deliberately interfering with a community using sock puppet accounts. What I think OP is talking about is a real user using the platform in an essentially honest way, but which happens to involve downvoting all the posts from one community. There could be a few reasons behind that, such as the example OP described of a user who actually has no interest in ever seeing the community, but doesn't know how or doesn't think to block the community. On all other communities, their behaviour appears totally normal.
  
  Ah. I don't think I've encountered that type of user yet. For the users I described in my comment, I give them a full instance ban. But In the case you describe, I think it'd be appropriate for a mod of the targeted community to ban them, but otherwise leave them be.
- one user who was mass downvoting to, according to them, mark posts as read
  Wow, I've not heard that one before. It does seem fairly ridiculous.
- Same feeling here
- My one school of thought is that popular posts at the top of the frontpage deserve to be downvoted in order to refresh the content.
  
  Why not just sort by Hot, New, or Scaled instead? The popular posts in Active sort will naturally go away as discussion in them dies down (comments boost posts too).

It's situational.
By itself, down voting but not otherwise interacting is fine. Hell, one of the uses of votes is to let the sorting work, so down votes are a valid form of interaction.
But, some people abuse that. So, if the pattern of their voting shows that they're targeting a single user, or trying to manipulate the ranking to shift sorting artificially, or points to some other bad actorship, mod intercession would be a valid choice.
However, a ban would not be an acceptable first intervention. That would be overreach, going beyond the scale of the issue. Contacting the user first would be the appropriate step. Remember, lemmy is still small enough that we can at least try to treat each other like people instead of just words on a screen.
Let me use a personal example to illustrate why contact first is the better option.
There is a community, which I won't specify to avoid causing them grief, that is dietary based. Because of my background, I have a higher than usual grasp of the general subject, and have higher personal standards for health claims surrounding fad diets.
As such, when I would scroll past posts on that community that were factually incorrect, or didn't give supporting evidence outsider of a YouTube link, I would down vote it without any further action or interaction. Early on, the only posts were being made by the mod of the community, and they noticed that not only was I down voting almost everything on the C/, but that was all I was doing.
On the surface, that can look sketchy, right? Some rando just down voting with no observable pattern.
So, they contacted me. Asked what I was doing and why. I explained pretty much what I said here, and the conversation was pleasant. That started adding in text that gave more info than just linking to a video, which means that people scrolling by weren't just hit by what amounts to ads for a fad diet, which is a major problem that isn't really discussed much, but can have massive effects on people's health.
With the extra effort in place, the posts ceased to have that same quasi-subliminal effect where people just absorb it passively due to it being background noise. So, I no longer needed to down vote those posts, and ceased doing so; reserving down votes for posts that either weren't on topic for the C/, or contained things that amount to disinformation.
That interaction gave me a ton of respect for that mod, even though I still disagree with what their community "advertises" based on my knowledge out nutritional best practices. It also made me willing to check their linked videos on occasion to see if maybe my knowledge and current best practices should change to incorporate that diet as practical and healthy. So far, all I've seen is that it's less harmful than I thought, but that's going too far off topic.
Looping back to your specific quandry, I think that it's always worth trying to communicate. Lemmy really is a unique social experiment, and the best part about it is the people. As long as we all try to handle things person-first, we have a high chance at things staying more like a community than a disconnected bunch of user names that might as well be bots for all we care.
Now, being real, humans are assholes. So chances are that your attempt at communication will fail, and may fail with them being a giant, gaping, stinking asshole. But I think it's still best to make the attempt before moving on to other options.
If they're an asshole, then you ban them on grounds of community interference. If they aren't, then maybe something good comes out of it.
Bans like that are a valid and useful tool. Anyone saying otherwise doesn't understand what it takes to keep a forum running smoothly. But it has to be a scalpel, not an axe. It's way too easy to slip into being a power tripping bastard that ruins the very community you're trying to keep healthy. If you find yourself reaching for that ban hammer before trying other things, it's time to get some backup. Find someone to help take the load off, to spread the stresses of having to police a community and yourself.
- Well worded.
  Just to add to your example (which is nice by the way, quite good to see such interactions happening), I've seen the opposite, when you identify an account with literally 0 comments, not even just on the community, but on the whole platform, and who systematically downvotes everything, the will to reach out to them might be low.
  Because, let's be honest, as you said, the place is small, and if I were to find you downvoting every post on one of my communities, I would reach out to you. Even if it's someone with just 1 comment a few months ago on a completely unrelated community. But if it's an account as I described above, I'm not sure if I would hold a grudge against someone banning them.
  
  Oh, yeah no way would I be mad if someone felt the need to just ban and be done. I've been on the receiving end of zero communication bans a few times tbh, and hold no grudges about it. Sometimes a mod just has to handle business and move on, leaving it on us users to put in the effort to appeal. That's a valid choice imo.

PTB. This thread is insane to me. If one person is downvoting posts, so fucking what? Just because they only downvote means their opinion is less valid? Maybe the content just sucks. If this is banworthy, then I might just be done with Lemmy already bc this is some petty bullshit.
- Maybe the content just sucks.
  If the content on that community sucks, why doesn't that person block the community?
  I don't like politics, especially US politics. There is too much of that content on the platform, and it drowns all the other type of content.
  I blocked all those communities and moved on. I don't downvote all posts on those communities because "it just sucks".
- If someone comes across an active well moderated community filled with on-topic content of an objectively high quality where everyone seems to be having fun, and that someone downvotes everything in that community every day, how is that beneficial? Why don't they post content they like? If they hate everything, why don't they block the community? Why are they spending time and effort to downvote so much stuff every day when it would be easier, and seemingly better for their mental health, to either block and move on, or contribute the flavor of content they want to see?
  No one is getting banned for downvoting content here and there, but if they're putting effort in ensuring everything in a community is downvoted, they'd just be spreading illwill for no good reason :(
  A good mod will help their community grow and flourish and have a good vibe. A mass downvoter who contributes nothing else is harmful to those things, and it makes sense a mod would want to defend their community against that.
  Edit: kind've ironic that you simply downvoted both me and Blaze 😅
  
  Edit: kind’ve ironic that you simply downvoted both me and Blaze 😅
  Indeed ha ha

Ask your community members, maybe?
I lean towards "go for it! block away" but I come from a different internet culture than reddit so could never mod anywhere similar. You might get better advice from someone who's spent more time in these spaces.
To me, It seems like chronic downvotes from nonmembers could mess up your comm's discovery. And is mildly annoying. Not terrible, but I don't think they need to be evil to get a community block. If you do it too much, your community might stagnate and people might start a new one. Oh well! Good luck to the new guys, imo. You might end up on PTB and get harassed. Always a risk as a mod, unfortunately.
Put "frequent exclusive downvotes from non-members gets a block to increase likelihood of community discovery by people interested in this content" on your profile. I mean, if you talk with your community members and they agree it's an issue.

Immediate ban seems overkill. If I were a mod, I would ask the user privately to explain why they are doing this, and decide based on the response. This also puts the user on notice that their actions are being noticed and watched, which itself might cause a change.

~~It’s not possible to see unless you’re an admin or use some ActivityPub service that federates with Lemmy but doesn’t hide this information from the end user.~~
That is to say, I would if I could, if there was a malicious pattern. I didn’t understand the outrage that Reddit wanted to do that too (although their ruleset and ways of enforcement are not my cup of tea).
In the meantime, lead by example and refrain from downvoting unless for rule breaking content or offtopic so that toxicity doesn’t increase.
- As of Lemmy 0.19.11 moderators can now see votes.
  
  I’m on an instance with 0.19.11 but can’t see it anywhere. Are there any instructions on this?

Is it possible they're a bot?

If it's everything (in a community/profile) maybe, particularly if they don't have any contributions elsewhere. Or if it's actually something heartfelt/personal I can see being more touchy about it, less so with bot-like aggregation or reposting old content etc.
Because otherwise, it's not going to be obvious where the line is when you add in time and % of content. If you're using a bot/script to detect voting, you're likely going to ban users just for casual browsing (even if they could explain their votes). Especially as an instance ban purges a user's account (to users of that instance) as if they're on the same level as an SEO spammer. Does a voting disagreement mean the rest of their account is invalid?
I don't know if it's fedi incompatibility or just 0-transparency moderation, but dubvee might be doing this but it's unclear as they seem to be just handing out silent bans (no ban reason stated).

PTB?
- "Power tripping bastard" = a mod abusing their power, usually by banning people they dont like personally instead of bans because of actuall rules violations
  
  Thanks
- "Power-Tripping Bastard"
- !yepowertrippinbastards@lemmy.dbzer0.com

I have strong opinions here. I view moderating a small community as trying to grow a garden and set the tone such that other people feel comfortable contributing to that garden.
The core problem with negative only participation is it makes the community hostile to new contributors. Most people on lemmy are lurkers, and if they feel that their post will be met with overwhelming negativity they simply wont post.
Downvoting is a form of participation, its a negative signal by design.
If someone hates a community so much they feel they need to downvote it every time they see it, but they don't want to block the community, its totally reasonable for a moderator to help them block the community so it doesn't ruin their lemmy experience (i.e. ban them from the community so they don't see it anymore)
- That makes sense

If their only contribution is downvotes, then it's not really a PTB IMO and should be done.

While lemmy is still small I think its worth asking them what's up. Is there a timeout/suspension function? Suspend their account for 30 days.
- I dont think Lemmy has a timeout function. Would be great though
  
  You can ban people temporarily

Have you tried messaging them?

How do you see who is up and down dooting posts?
- It's public information available for instance admins and any sufficiently motivated person who knows a bit about the ActivityPub protocol.
  
  Mods can now see votes directly from the Lemmy Web UI, from version 19.11
  
  Makes sense lol cheers
- You don't seem to be a mod 😅
  
  Solid observation hahaha all my communities were created from my account where the instance shut down so I lost them
  
  Wait, can mods see who voted on posts? I thought only admins could!

If it's only downvotes, then a ban is justified.

Yep, they get banned. End of story. Though I'd be willing to unban them if they appealed and explained their behavior. Or if there was a mistake on my end, obviously

It’s not considered powertripping as that is a form of vote manipulation.

I'd say just ban them. Doesn't look like a healthy relationship for either side. Worst thing that can happen is they complain and you get to know the reason for their behaviour... And if they're under the impression it will get rid of the posts, banning them will accomplish that, so win-win.

If they only downvote everything, I think they deserve a ban.