Lee brought something important to my attention regarding SFS:
http://www.vbulletin.com/forum/showt...=1#post2104854
SFS is very successful at blocking spam bots from registrations. But that causes another issue. These spammer registration scripts can try to register again in millions of a second. They keep banging on the door. Hitting the server many times and causing high server loads. This is also why SFS logs show so many blocked spammer accounts.
So while SFS resolves the spam problem it relocates the problem to server load.
One solution could be to analyzing the http fingerprint of blocked spammers and blocking them from the site. Or even adding a temporary IP block for those IPs.