Perhaps we need a jerk flag. If enough people flag a user as a jerk for making inappropriate comments, then that user would be banned. People could upvote the offending comments all they like, but after X many jerk flags, that user is no longer a HN'er.
Edit: You could say that only experienced users (members for several years with X many points) may use the jerk flag. Or something like that.
My suggestion is to get rid of down-voting. Griefers that want to abuse the system would need to do a lot more work if their only tool was up-voting. Accentuate the positive!
This also reduces the problem of people down-voting posts simply because they disagree with an opinion.
Edit: You could say that only experienced users (members for several years with X many points) may use the jerk flag. Or something like that.