I agree that the private message was very inappropriate, but the best response to this type of thing is not to let it bother you. You can't fight people that send messages like that. They'll just find another computer and hit you again. If the damage was really severe (ie lots and lots of negative posts diluting the board) I could take some action. Think of it like someone yelling at you from a passing car. It's certainly bothersome, but chasing them with a baseball bat is only going to make the situation worse.
You're all right about IP addresses. Yes, I can do a reverse lookup and see who people are, provided that they have a static IP (and this person does). At the same time, so many ISPs these days assign a new IP with each connection so banning an IP might (a) not be effective and (b) might have the counter-effect of banning the innocent unlucky user.