I quite like StackOverflow's solution to this problem: You can only downvote if upvotes >= constant * downvotes, that is, if you have made a few times more upvo

I quite like StackOverflow's solution to this problem: You can only downvote if upvotes >= constant * downvotes, that is, if you have made a few times more upvotes than downvotes. Clearly, if you can't find at least two comments you like for each comment you want to downvote, you're not really trying.