http://www.vbulletin.org/forum/showthread.php?t=248042

This is now active on the forum, and the settings are located in the admincp. We were having a few spam accounts sneak it. Although not able to fully register, they did create new accounts, but obviously didn't verify their e-mails.

Anyway, give it a go, see if it works, or if it breaks things, feel free to turn it off.