The only way to prevent scams with high effectiveness is for Theymos to ask the campaigns to do extensive KYC, which I'm pretty sure he doesn't want to do simply because it goes against the cryptocurrency ethos.
Some yarn:
If scams and legitimate services occupy the same space, which one do you think will saturate the market?
Assuming the same amount of effort, which has a lower barrier of entry? New scam threads, or new service threads?
Considering that users can be "downgraded" to red trust but are always able to participate in the
scam economy, what will users do with the larger market?
Considering that new users are (effectively) no different (+/-) from users with red trust in terms of reputation, what is the largest possible market?
When competing against other signature campaigns, would it be possible to cut costs by scamming the participants as well?
Given the amount of time it takes to generate a significant reputation on the forum, and given the risk compared to using the equivalent number of red-trusted accounts...
Would it be better to constantly prop up scam signature campaigns for scam sites, so that you can get something closer to 95% return (based on total deposit/input) as opposed to regular means? Direct theft is more profitable than starting a real website, and it's far easier to set up: no need to worry about long-term considerations since you're going to close shop very quickly.
Under these circumstances, Bitcointalk still is occupying a relatively peaceful time: scammers have yet to maximize forum exploitation.