This is how it is at WC3C and it works wonderfully. I believe that -1 is fair for minor offenses, -1*N for the Nth offense, and -(arbitrary number) when it isn't some standardized breach of rules. (ie. Case by case)
Here, though, because of the magnitude of people shelling out rep for no good reason (At WC3C, it's actually very difficult to earn rep), those numbers would be scaled up. Actually, it'd make the most sense to scale the amount of negrep to the positive rep of the user if >0. That way, users who have high rep get hit hard because they should know the rules by now. That way HT would lose like 20 every time he acted like a moron.
Also, my point stands that the staff needs to arbitrarily know when a user has screwed up enough to merit a ban. Seriously, I'm quite sure that the staff is capable of that reasonably.