Like Alex said, a hack isn't necessary.
But anyway, disabling bad rep kinda destroys the cycle. You get good rep for posts others like, and vice versa for posts others dislike. If people want to abuse that, it's their problem.
I might as well offer my own suggestion... Members of the main usergroup are unable to give rep. However, they can post in a subforum under New Users/Welcome (or somewhere else) after they get a certain amount of posts, and ask to have rep giving enabled. Should somebody who got rep find it abusive, they could contact a higher staff member, and if the staff member agrees, the rep gets deleted and the abuser gets moved into a different group of which the members can't give rep, so the staff know they abused rep.
Sound good? XD