They always hammer new forums. Give clear indication it's moderated (by wheel's #3 advice) and most of it will die off in a short time.
Good advice, but with both of these enabled, spammers will still use your forum as a "test bed" for the captcha-breaking programs out there. Had it happen.
In addition to these two, what really stopped the captcha-bots was adding a custom registration field with a trivia question. You set the custom registration field to NOT appear in the profile, so in effect it's a validation field only at signup. "What is 6 plus four?" You have adequate controls to explain the question and put it last in the registration form.
I had to change this field over a period of 2 weeks or so, as they kept coming back and manually changing the values for their bots. After that it just stopped. Completely.
I've never had to ban by IP (yet,) although I HAVE banned by email pattern (.ru, meds, pharm, etc . . . )
Last is get registered at and dig around the vBulletin forum itself, lots of useful contributions there for just about anything (including this solution.)