The thing about systems is that we tend to think of them as if people are not a part of them. But people are a part of them.
One thought I would suggest is that it might be worth giving people with very high karma (maybe high enough that only a few people have this) an ability to post highlighted reprimands, and an automatic banning of a user if he or she gets more than, say, 5 of these in a month. These could then be reviewed of necessary.
One thing I would think about is that rather than having a system where computers try to control people, have a system which empowers the community leaders to set standards of interaction.