I have a suggestion if you are interested?
A while ago I was having trouble with bots on an invision board I used to have and I added some code to check stopforumspam.com for known spammers IP's, email & usernames when registering (After captcha validation) and if in the database they would be denied registration, and IP, email & username added to the ban filters and it worked really well
There may be a mod already available for VB which does the same or if not then if you need any info on adding this here then let me know?