rtcvb32: I don't see why it couldn't be added...
But limited posts with low rep might help, making them wait merely means they will make more accounts so when one goes down the next is ready to take it's place...
I'd hate to require that they have bought something from the store before being able to use the forums, but that could work too... As the likelihood of you needing help on one of the free games seems unlikely...
Encoding such a thing isn't easy. Limited post count for new accounts could help. Auto informing admins when new accounts post a lot in a short time would help too.
Having to have bought something blocks new potential customers from asking questions first before buying anything.
I would also prefer to leave the spam account enabled for a while but disable it's posting abilities. It's most likely an automated bot, so disabling would result in the creation of a new account. Keeping it enabled might prevent that.
Forcing some kind of human interaction to activate an account wouldn't hurt either.