I help admin several other vBulletin sites sites so I'd like to offer some advice on stopping the spammers.
- Turn on the "Verify Email address in Registration" option. This will keep the spammers from posting unless they give a valid address. You can also add legalese to the verification message that they will pay $10,000 for each spam sent. True you have to find them but at least they have agreed to NOT SPAM and they will PAY if they do. This will standup in court.
- Turn on the "Moderate New Members" option until they have posted at least a couple of times. This will allow one to "gauge" there intentions before you let them run free on posting.