Assuming here as I don't run this forum, but the way it works on most vBulletin setups is when you 'agree' you add to their rep, when you 'disagree' you lower their rep. There are limits as to how much you can plus or minus rep one person - e.g. if you neg-rep someone you can't do it again until repping someone else. You also can't neg rep someone when you yourself are in neg rep. I have found this feature to be useful as it is typically easy to figure out who the trolls/asshats are.