Meh, it would be sort of censorship, and against everything this forum stands for; Free speech. The forum doesn't really moderate scams, and I don't think limiting a scammer of creating self moderated threads is fair. Creating a new thread, and referencing it us probably good enough in the majority of cases I think.
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Plus they'd just use alternate accounts. That's what's happening already all the time. That would only make it harder to spot the scammer.
A self-moderated thread by someone who is suspicious or who just wants to create a bounty campaign is an instant red flag for me.
Usually, you don't need self-moderated threads unless you aim for a clean thread which is usually done when the topic is either complex or you have some specific posting "rules". There really is no reason to make a bounty thread or ANN self-moderated. If someone spams, report him, if it's viable critique, live with it and either reason with people or ignore them.