Forums and blogs will have logs of the IP address used on each connection. Get that, find which system the IP address is from, and that system will have logs that identify exactly which user did it.

This requires cooperation from two separate operators, neither of which will usually release such information unless requested by the police or a court order.

That is, the issue has to be serious enough to involve criminal investigation or (successful) civil procedure. No ISP or forum host will help trace mere "unpleasant comments".

You should be able to work out who the person is from their writing style, but obviously that doesn't prove anything.