[RyJones]

This is one of the reasons Begg Knives shut down his forums. We literally couldn't delete accounts fast enough.

[ValentineC]

When I used to manage web forums, I ask a hard question during signup and put the answer within the question itself (e.g. "hint: the answer is xxx").

This usually stops simple spam bots that are aimed at that particular off-the-shelf forum software.