[Reeve of shinra]

I am not a lawyer and this is just my opinion ...

A subpoena would have to come from the court and be signed off on by a judge. Someones a law firm will send a strongly worded letter to try and obtain information so be careful of that.

If you do receive a subpoena, you can see what they are requesting and discuss it with a lawyer. You can generally get a consultation fairly cheaply and the state bar association (google it) can give you a recommendation for a lawyer in your area that specializes in this.

Since you didn't receive a subpoena yet, you can't really do anything yet. Especially because you don't know what it will or will not request.

If they asked for user information, you might be able to dispute it and put the onus back on the plantiff by showing what slanderous remarks were made on your site and, assuming there are none, fight it on the grounds that they do not know for sure that its the same people on both sites and not just a common name. That would be a paper thin argument though and a legal fights could get costly on your part.

Whatever you do, don't do anything stupid like make up names as Pant suggested -- you are obligated under law to provide the information if it is requested via subpoena and its not worth going to jail for tampering with evidence.