I tend to agree Craig karma is not the answer nor is editing posts.
you could use a "three strikes" system -semi automatic warning
sent by e-mail case specific moderator e-mail - banishment for a period that depends on the severity of the event.

Could you add a moderator enabled flashing red politness warning?

The reality is that as in all aspects of life on the forum you are the sum of your posts in the "eyes" of the other members. Once you tarnish your "image" you never recover. Unfortunatly most offenders will probably repeat just as in other settings offenders repeat.

Some forums have an ignore button. I think it not a good thing as it leaves holes in the continuity.
Regards
Terry