My main suggestion would be introducing some more complex criteria - e.g. a minimum length (so “great post” doesn’t count) and additional points for longer (and presumably more meaningful) comments. I also set a limit on what each person can score for each post so that if 2 people just have a long, unrelated conversation, their score gets capped. I do something similar to this to review World Of Xpular’s top commenters and it filters out the spammers without manual intervention or blacklisting.