After receiving an upvote it should get exponentially harder to give more. Like votes 1-10 you have to double click; 10-20 triple click... At 100 you have to do a captcha. Just keep adding obstacles.
Ok but the idea is based on countering bots and bot nets. The only thing it really changes is the threshold becomes more difficult but the difference between maybe a 601 and a 602 is astronomical.