Spam Management 
vBulletin Options > vBulletin Options > Spam Management

vBulletin Anti-Spam Key Powered by Akismet
Enter a vBulletin Anti-Spam service key to enable scanning of user data where supported.

You can get an anti-spam key here:

Spam Scanning Post Threshold
This setting controls how many of a user's posts will be scanned by the Anti-Spam Service. Once a user's post count exceeds this threshold, his or her posts will not be scanned for spam content. To always scan posts set this value to 0.
Tom McIntyre 18th Apr 2009, 10:17am
The Akismet process includes reporting on the results of spam detection with false positives or valid spam detection. The process needs to be noted here for completeness.