The way to put a stop to that is to require a certain number of SUBSTANTIAL POSTS; actual participation. But Okuma doesn't want that, he wants an open classifieds section.
This would be easy to moderate too, and would also prevent the mass PM from a scammer, which is the biggest issue, and the hardest to deal with and catch. It's totally up to one of the users to report the suspicious PM before we even know it's going on, and dozens of PM can be sent out before it ever gets reported.