You could always ask newly registering users what their level of certification is, and from what agency.  (There would probably have to be an option for aspiring divers, as well.  After all, I was an enthusiastic newbie just a couple months ago.)  Prospective new users who have absolutely no clue about diving and no interest in it (read: spammers) could be stymied by questions like this.

I think there are some spammers out there who make programs that can be used to automatically register for forums and post spam articles.  I saw it happening on a message board I run for my inline skate club.  I removed the registration links from the message board, thinking that nobody would be able to register, and then found that spammers were able to register anyway by directly using the registration URL.  Those programs even have a way to defeat the "type in the text shown in this graphic box" thing.

I guess the spammers will do anything possible to sell just a few more bottles of fake Viagra and weenie-enhancing patches.
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Addendum: After a brief search, I found that there is indeed a piece of software that will post spam to various message boards running software like phpBB, Invision Power Board, vBulletin, and others.    Feh on them.