What's the best response to someone who believes in hard determinism but also uses this to deny responsibility for any immoral actions they commit?

Punch them in the mouth, it's not like you have a choice.

"What have I done? What have I done some more?? What have I continued to do??"
- "Guys, we're in some big trouble here...."

You punch them in the face, and then tell them they can't be mad about it, because it's not your fault, it was preordained.

Smack em, assert the same statements. If they argue, smack em again and repeat.

... that's a paddlin'...

I'm a fairly hardcore/radical determinist, and tend to agree that individuals shouldn't be held morally responsible for actions, any more than a hammer is morally responsible for driving a nail. However, that does not mean people should be free from consequence. There are plenty of reasons - even as a hardcore determinist - to hold people to account for their actions, either as a social corrective mechanism, public safety, deterrent, or personal sanity.

As for getting their actions to align with your morals, that's a more complicated question that depends on the type of person they are.

This is a great answer.

Just like someone's immoral actions are preordained, the consequences are too.
How does a hardcore determinist believe in “shouldn’t?” Doesn’t that imply that people have the ability to change their behavior?
- Doesn’t that imply that people have the ability to change their behavior?
  
  My answer changes depending on your meaning but:
  
  Of course. My brain is constantly updating and improving itself. I'm just not ultimately in control of how that process happens. Though that does not mean that I should stop living. I can still experience and enjoy my life, and 'choose' to improve it. It's just that the I that made that choice is a consequence of my brain calculating optimal paths based on a myriad of factors: genetics, culture, circumstance, biological drives, personal history, drugs, etc.
So..um what drives the hammer?
- What drives the thing that drives the hammer? What drives the thing that drives the thing that drives the hammer? What drives the thing that drives the thing that drives the thing that drives the hammer?
  
  Physical processes out of our control.
- Well, I blame the nails. They're just asking for it.
I am genuinely and in good faith interested what you think about quantum mechanics and that there seems to be an element of true randomness there.

I was pretty much a determinist until an actual physicist that I know and respect told me that he is totally convinced that there is stuff in quantum mechanics that just cannot be predetermined.

And if anything can be undeterminable then by influencing other things there would exist true randomness and then a fully deterministic world cannot exist in my eyes.
But I am very willing to learn more if you know a good counter-argument since I always thought determinism is quite an elegant view of the world.
I just cannot follow it if I am not convinced it is true.
- Randomness doesn't really save traditional free will. A robot that selects its actions by rolling dice is not any more "free to choose" than a robot that selects its actions according to a deterministic program. There isn't any free-will juice that gets introduced by adding randomness.
  
  Your "free will" is the process by which you select actions. For humans, that's a bunch of physics and chemistry happening in your brain; it receives influences from your senses, your body, and its own self-awareness (i.e. its model of you, your actions, tendencies, etc.). Whether that process depends closely on QM, or is boringly classical, doesn't control how "self-determined" it is.
- One interpretation would be Many Worlds; that is, every quantum possibility is real in its own multiversal branch. So, to assign moral agency you would need to show that I chose the world I'm in now, over some other version of my life in which different choices were made. Although, I'm not certain you even need to go that far: I have no idea to what degree quantum randomness can actually affect our choices. But, in any case, that too would be out of our control.
- There are layers to the universe. While you can't predict everything you don't usually need to. The object is dropped and therefore it falls. If you zoomed in deep enough you would see the chaos that is going on in the subatomic but the object still falls all the same.
  
  Not being able to predict everything does not mean we can predict nothing.

"Ah, then my decision to shun you and tell everyone I know to do the same ... that is also preordained, and you mustn't hold me responsible for doing so."

Well, they can't seriously be that stupid. It's proper 8-year-old shit in a veneer of "this philosophical thing I heard about once" - it's 100% the Simpsons bit "I'm just going to windmill my arms and keep walking forward and if you get hit, it's your fault". Laughing at it seems like a good option and I personally would probably hang out less with whomever.

well then their punishment was predetermined too

What's the best response? The best response is to laugh in their face and go find someone else to talk to.

The person you described is an idiot. Can you tell whether a person actually has free will by observing their actions? Like just by looking at them, can you predict exactly everything that they're going to do?

(This is actually almost identical a famous problem in philosophy called the "philosophical zombie.")

If the answer is "no", and it is, then it doesn't make sense to base your actions based on whether you have free will, because it doesn't actually have any effect in your daily life, other than to irritate other people with your pseudo intellectual babble.

Any claim that is asserted without evidence can be dismissed without evidence.

Any claim can be inverted, so lacking evidence in either direction, this applies to the inverse as well.

I personally prefer more psychologically rooted arguments that lean towards at least compatibilism. If a belief in free will, regardless of the actual fact, is sufficient to affect one's actions, is that not evidence against hard determinism?
- Dismissing a claim is not equivalent to asserting the negated claim.
- Nah. It just extends down. Your belief, and any changes over time, are also predetermined as some sum of your inputs.

Disown them.

Tell them they should have factored the consequences into their predetermined behavior.

They should also understand why consistent enforcement is necessary to prevent others from making the same preordained decision.

Even if the choice is only illusory, it's indistinguishable from free will in every other respect... So we will treat it as such

"Even if the choice is only illusory, it's indistinguishable from free will in every other respect... So we will treat it as such"

I like that!

A good response to someone who believes something is to tell him what you believe. Then maybe talk about it or maybe not.

There's an episode of the Good Place where they discuss this exact thing (well, replace "immoral" with "romantic", but still), and I'm pretty sure the motivations are the same. They don't actually believe in determinism as much as they claim, but they don't want to be responsible for their actions and determinism is a good excuse they can use. You can't use logic to get them out of this belief, because it wasn't logic that made them believe it to begin with.

There isn't, someone set in their ways like that won't change so don't bother trying.

Just pitty them because anything they accomplish was not because they tried.

Let him read the following dialog between God and a mortal considering determinism. It's actually not very theistic, but merely presenting the free will problem in a logical manner.

It's by logician Raymond Smullyan and it shows how untenable the position of extreme determinism is, without polarizing.

It's one of the things everybody struggeling with the free will vs determinism should read.

https://web.mit.edu/people/dpolicar/writing/prose/text/godTaoist.html

I was preordained to not read this.
- Not really, you're just being obtuse, that's ok.

"Damn that's crazy bro"

“Yeah mate you’re just being a cunt. You do you tho and see how far that takes you”

It really is too bad the universe forced me never to talk to you again though.

There is a difference between free will and agency. I don't think humans have free will, that requires the supernatural to be a thing and I don't see evidence of that. Agency clearly does exist or else you would have to maintain that a billionaire and a slave have as much control over their life. Your friend is confusing the two.