Alas, they're like fleas. Go look at the QtWeb forums, where they seem to have given up scratching
I agree with siria, pre-inspection by a mod is needed, or rapid action when people report the threads.
OTOH, we assume it's real people making the posts -- what if it's only real people signing up? Maybe all we need is CAPTCHA for both phases, and some equivalent of a "hosts" file to block the IP?
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