Yeah, like Lynne says, if you haven't already, make sure that they are new registrations on not old ones that never posted. As for the logs (assuming they are new registrations), I was thinking get the registration ip of one of them, and then search the web server access logs to see which scripts they accessed.
ETA: Also if you haven't already you might try disabling all plugins. Maybe an addon is inadvertently allowing registration even though you have it turned off.
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