Source: http://www.simplemachines.org/community/index.php?topic=277813.msg1822406#msg1822406

Great spambot killer mod for vbulletin forums and can be downloaded at vbulletin.org. I am currently using it on a Vbulletin forum and seems to be working great.

Is this possible to incorporate this into SMF?

You can go to this page and see what it is doing live.

http://uspam.pagerobot.com/uspam_genstat.php

Thanks