There are also hacks you can install for vB that flag words such as WoW Gold that are commonly used by spammers that instantly end up in a moderating queue. This way none of your members actually see the spam and bots can't index it or count it as a backlink.

You can then deal with these spammers as you see fit and no one other than you and the staff team will even be aware of the spam.

I'd also suggest you lock down your Private Messaging System so users cannot send more than one or two messages until they reach 10 posts. Most bots never make 10 posts but it also keeps nefarious users at bay who are only interesting in spamming your members via PM.