This is just a thought (and maybe not even a possibility in vBulletin)...

Instead of allowing people to rep negatively, is it possible to revert to using and then leaving it up to the moderators/admins to determine whether or not a negative rep is justified? With a few (not alot) of the casual perusers that come across VBF, it seems like you have a better chance of getting a negative rep because what you have to say; may not be what they want to hear.

(Side note: This doesn't pertain to any thread specifically. I've seen/heard about this happening a few times and have been on the receiving end of this in the past.)