You need hacks that MODIFY the registration page. That's the door they use. There are some script that "scan" the registration page and just go over it. If you have some new required fields they cannot register, thus, no spam. I had many spammers register on my VB too and only with such a hack I was able to stop them at last
How do you enable the feature that forces to admin to manually accept a user before the new user is allowed to post on the forum?