I don't agree with restrict users from opening self moderated thread. Users many times try to post off topic replies and spam sales threads by asking silly questions and to stop them from posting, this option of self moderation is given which is not given on any other forum.
Scammers would continue scamming even without self moderated threads so that's no an issue as DT members leave trust ratings on such accounts.
Beside, scammers tend to either do quick impulse/pressure scam or obfuscation, lying in wait. Meaning they will either post in another's thread or send a PM trying to push the other person into sending BTC quick.
Or hide that they want to scam people by creating a sale thread
and locking it.If they self mod and delete comments against him, he is going to get red trust *really* quick since its easy to justify red trusting someone that delete your posts when you tell everyone else he is being scammy.
Meanwhile if you don't say anything and the thread is locked and you just ignore PM's of people that want to use escrow, its much harder for others to justify giving you red trust.
So a better thing to do in this area would be to prevent new users from locking their own thread. Negative side effect aside, that would accomplish what OP intended, i believe, much more than self-modded.