Stop spam on your forum or blog

Stopforumspam.com keeps a list of IP addresses that spread spam on forums and, for example, WordPress. Modules are available for various scripts that use this information to keep spam out of your forum.

Check http://www.stopforumspam.com/contributions to see whether the script you use is supported.
PHPbb, vBulletin, Simplemachines, MyBB, PunBB, PHPFusion, XenForo, Elgg, Symphony, UBB central, Pligg, ExpressionEngine and WordPress are currently supported.

If you are reasonably handy, you can also easily build your own integration. See:
http://www.stopforumspam.com/usage and http://www.stopforumspam.com/code

At the end of the PHP example, add for example this:
IF ($arrXml['success'] == '1' AND $arrXml['ip']['appears'] > 0):
return; // or jump to a completely different site, show an error message or something else creative.
ENDIF;