It's a forum security idea. I've got an SMF installation on my domain too, so I'll trial my idea there, and if Shockwave likes it, he may trial it here. I don't know yet though, as he hasn't replied.
I'm trying to keep it top secret until it goes live, so as to scupper any hackers who may be reading the forum against our wishes...
One thing I've noticed about the spam users is that a quick Google of their user names often reveals their nature - if you see results on dozens of unrelated boards and the time/date of registration are close together, it's a bot, or an active spammer.
If they're on many boards, but the time/date of registration differs greatly, chances are it's a person who uses the same username everywhere.
If there are no results, it could be a new user, or a new spammer which has not yet been crawled on other boards by Google.