Bob, you're dealing with a troll. What he wants is responses. He doesn't care whether they're abusive responses or not; any answer proves that he's succeeded in annoying people and making them pay attention to him. Think of him as a network terrorist; it's not about his message (not that he has one), it's about his power-trip.
If you answer, you encourage him.
Don't feed the trolls. Just killfile the sucker, and/or the threads he's trying to start (more effective in a case like this where the twerp is munging his userID). If he doesn't get answers, he will eventually get tired of punching pillows and find an easier target. (After a period of trying to prove me wrong, of course.)
Or, if you think he's worth the effort, do a bit of header analysis and see if you can get his ISP to shut him down. Looks like he's mostly coming in thru the University-of-Berlin server... which does require user registration specifically so they can kick out abusers. (I'm using that server myself.)