You are off to a good start by establishing a history point _before_ he attempts to post a bad review.
Definitely post his number in the powder room. It would help to post an alert with a partial number too. (X-out the last 4 digits)
Alerts go here:
http://www.eccie.net/forumdisplay.php?f=35
Also don't depend exclusively on the Mods for help. One of my favorite providers was the victim of a fake review a few months ago and the Mods have left it standing. I could _easily_ prove that this review is fake by posting a few links. However, that act would "out" the provider, so I cannot do it.