Not at all sure of the legalities here, but I would definitely take a print-out of all that this "person" is saying about you. I'm almost certain that publication of defamatory material on the 'net is tantamount to libel or, if not, slander. By taking a print-out, preferably with a witness present, you have proof in hard copy. See a solicitor on one of those "£5 for half an hour" deals and see where you stand.
Of course ICQ are going to tell you that there is nothing they can do. A friend of mine went to a chat room (big mistake) and a "lurker" copied down his e-mail address during a conversation with a third party. The next morning, my mate's son, all of 5 years old and very proud of the fact that he could get Daddy's e-mails, was horrified to see some of the most disgusting filth appearing on screen. The lurker had logged in to one of those "porn in your e-mail" sites and given them my mate's address. The ISP said there was nothing they could do about it. Needless to say, my friend cancelled his subscription to that ISP PDQ.
This, though, is a whole different ball-game. This is not just a prank, but could be argued to be doing you harm, therefore you may have a case, and one that ICQ will have to acknowledge and do something about it. A letter from a solicitor might do the trick, as they would not want the adverse publicity.
Cost will be a huge factor if it goes to court, though, and it may just be wiser to ignore the whole thing. This sicko is only getting his jollies out of this because he knows you are upset by it. If you don't care to respond and can avoid getting riled by it, and that includes talking about it to anyone (if you talk about it, you are, by definition, dwelling on it) he might give up. Who hates you enough to do this? It must be someone who knows you quite well if they know enough to generate an ICQ account in your name. Answer that question, and you may be halfway there.
[This message has been edited by R O Tiree (edited 02 November 2000).]