You have a legal right to sue the person making the comment, when you iniate the court-case, your lawyer will contact rip-off report to obtain that person's name. Should they refuse to provide it along from their logs/membership then they will be libel for damages e.t.c. as they are then to be considered an accomplice.
The case recently where google was forced to reveal the identity of one of it's blogger so they can be sued proves its possible, its just the same as any other libel case, its a pain in the arse to go through all the hurdles to get it done. This case set a precident which basically means if you start raising the law suit against the person in question then their name MUST be provided. The law-suit could also be directed at rip-off reports as previously meantioned for refusing to take down the libelous statements. Its exactly the same situation as with newspapers really.