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Did you disable anonymous comments in Basic antispam plugin settings?

Basic users can't do any harm to your installation other than slowing down mysql operations (if you have hundreds of users)

The main reason you get those users is because spam comments from valid users look more legitimate than visitor comments.

Thank you. Considering the moniker 'da ruski mafia' I have to weigh how reliable is this source. ;o)

I really didn't know if there was some kind of backdoor they could exploit. So I'll let them stay users in the future.