Since the forum spamming is growing again 
I suggest the guys from scriptcase start to implement the counter measures.
http://www.vbulletin.org/forum/showthread.php?t=176481 shows a nice entry.
You can receive the datadase from http://www.stopforumspam.com/ at no costs. So this should at least make live easierā¦
All you would need to do is add it in is in the registration system.
The alternative is
https://www.vbulletin.org/forum/showthread.php?t=248042
and using akismetā¦