I've had such problems too. Thunder tried to join my server but reported that he was banned. AJazz RAD was faking Thunder's IP. I didn't seem able to ban him, though...

You are probably on some sort of banlist of yay iT, or not on a whitelist. Maybe only iT members can get in.

EDIT: which is untrue, as there are also members of other clans on the server (RR,CC...)