I find that this forum seems to attract spammers on a daily basis. From porn to drug sellers, they have a tenacious way of coming back, faster than you can delete their posts.
I participate in a few forums regularly and run one myself. I find none of them have the kind of spam problems that you do here.
It's hard to believe that you can't make changes to the registration process or something to stop (or at least minimise) this from being a regular forum feature.