How many people do you think wander into large forums with the expectation, "I need to be on my guard or else I will be scammed, because the forum allows scammers to stay here."
Maybe big red warning on every trading board post started by a newbie; something simple like "Scammers flourish in a free marketplace, WATCH YOUR ASS!" That might get the job done, but then again it could get ignored just like all the other giant red warning signs.
At some point one has to accept the risks of living in a free society. We can police our own in the hope of preventing repeat offenders, and sometime we can't even do that. The only alternative is regulation and enforcement, and I doubt theymos will ever venture there. Personally, I would rather accept the risks of a free forum than the alternative.