Hello all - please pardon my interuption on your forum.
I stumbled across this site/thread while searching spammers of my own forum.
It seems we are in the same boat and having trouble with several of the exact same people.
One thing I have discovered you may be interested in: there are entire "spammer boards" (for lack of a better term). These individuals visit the phpBB and vBulletin help boards, plus search terms like 'message board', 'forum', etc. to find our sites. Once they find your board they post the URL on their site and you get another wave of spammers coming through. The more popular your site becomes, the more visits and problems you can expect.
What they are hoping to accomplish is that their URL gets picked up by a search engine bot when it crawls your site. Your popularity gives more credit to their link and boosts their spam site.
Seems like a waste of time to me, with little chance of actually working but that doesn't stop them from trying. I am currently getting anywhere from 30 - 50 of them each day.
Basically, I just wanted to offer you a heads-up and tell you to keep doing what you're doing - visual and e-mail confirmation. I have already tried the probationary period, it didn't make any difference. They behaved for the first couple of posts to gain access and then cut loose.
If you would like to double-check any new registraints with me that you may not be sure of, I would be happy to share that info and save you all some of the messes I have had to clean up.