Sorry to ask this
Why do you need email verification for registration?
People can easily create 10 or 20 email from mail services, like a fake email address!
Verifying their email doesn't stop them from SPAMMING the forum
What we can do, IMO is to capture their IP address (I am sure vBulletin have this feature/Mod). If they are CONVISTED to be SPAMMING the forum, just give them warning that will leat to banned for life if they still SPAMMING the forum
Cheers...