Yeah, a simple methode for spammers is to simply register themselves, as humans. Then bot as many posts as the can before getting knocked off. Obviosly, this has been tried and results in large amounts of spam.
If a spammer can make money on a spam messages, then spamming will be done. Someone, somewhere will benefit. Besides using automagic tecniquies, which can all be broken, there needs to be a cost associated with it that allows the registering party to have some confidence that it is MORE expensive to register and spam than simply ignore the site. Using "slave" labour means no cost. If he has to pay, he won't.
Even delaying him by moderating his posts really is not a solution. It is only another delay. In any group of ten people, chances are they can type ten reasonble messages to get by this moderation...then spammmmmmmmmmm.
I would suggest a refundable cost, or garantee, when registering. If you start spamming, it is forefit. Or a membership fee, as is now in place. Anything that makes the spm more expensive than not.
Just another thought to add to the mix...