In addition to the advice that you have already been given, if the person in question is really determined to keep on sending you messages and tries to contact you by sending you a PM via the forums, click on the message and then when it opens up you'll see two blue boxes. The first is to add that person as a friend - ignore that and click on the second one, which is the one to use if you want to block that person from being able to send you PMs via the forums. That way it should make it even harder for this person to contact you via eBay, and if his/her efforts to contact you always end up failing then it ought to kill of any incentive that he/she may have to try to contact you, in which case you will more than likely end up hearing nothing more from this person.
