Don't you think it would be better if only moderators, or staff could use that feature? It is pretty obvious that people will spam it if not. If not maybe you could add a condition that people can't give more then 3 karma per month. I think this would be sweet.
How about a new tool that would oblige you to give areason for each Karma? You could add in the profile of the member why each member got a karma from who. That way, moderators could see if anyone abuse .