I don't see the value in responding to him in a serious manner. He's an idiot.
Nobody is holding a gun to your head. If I agreed with your DSM assessment I'd surely have him on Ignore.
But I think he's just behaviorally disordered behind the keyboard, and in reality most of the lights are on and there's someone home. As long as I occasionally communicate human to human with him, it's working okay for me.