Forum Moderators: rogerd
Libel is a big issue on forums, you can guarantee it will eventually come up once you get busy enough. My position is not to allow posters to discuss particular companies and websites if at all possible. It's drastic, but libel can be expensive if you don't deal with it well.
The best thing you can do to prevent problems is be vigilant and educate your members. Some of them may not even realise that libel action can be taken against them or you for their forum posts.
What action have you taken against the member? Was it a written warning, a ban, anything like that? The trouble, from your perspective, is that this is just an allegation of libel. There's no way you have the time, as a webmaster, to get to the bottom of the issue and discover which one is telling the truth. So don't take sides and give up the IP unless the police or courts ask you to.
Even in a managed corporate network, when I'd get a request to pull "Bob's firewall log" I'd stress to the requesting party that this log reflects connections made from the machine adminstratatively associated with Bob. No one can conclusively assert it was Bob who made any given request, but if there's a pattern of requests over a long period, you're free to draw certain conclusions yourself about the likelihood of it being someone else at Bob's machine.
Of course, the poster's IP was 127.0.0.1, wasn't it? ;)
IP info isn't always useless - sometimes it leads back to a home connection, a business or a school, etc. In conjunction with the time of the post, it may be possible for the ISP or network administrator to identify the PC or user that made the post.