Spam and IP banning

[MrWookie47]

The micro forum (and probably everyone else) has been getting a rash of spam with the following text:

[ QUOTE ]
Playing on UB? get 31% rakeback. www.instantrake.com/faq

[/ QUOTE ]

I've been banning the users one at a time, but since it's all the same site, it's likely that it's all the same person. I haven't figured out how to look up the IP address of a user, though. How do I do that so I can ban an IP, or has someone already looked into this?