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Have you installed Protector 3.04 with RBL plugin enabled?
Also, do you allow anonymous users to post comments, if so then disable it and only allow registered.
The RBL plugin will check RBL databases for IP addresses which are known to be spammers. The owners of the IP addresses have to contact the RBL owners to get their IP addresses removed.
Also, when the user signed up you may have got an email from your website (if you had it turned on in admin) saying this and it will contain the IP address of the abuser which would be found in the emails headers (only ban this IP address if you are sure that this is the abusers IP address).
