Find a few examples of comments that are unambiguously (a), (b), and (c) and have either you personally or someone you trust flag them as such. Next, take the set of all people who upvoted the abc-flagged comments. Their votes now have a 50% chance of not counting towards vote totals from now on, but in a way that the user isn't shown that their votes aren't being counted -- perhaps with an artificial "offset" vote that appears a few minutes later.
There's fun parameters one could throw in there too, like exponential decay on the likelihood of a vote being magically offset that spikes back up every time the user votes stupidly.