I've come across folks like this in religion and politics. I learned long ago that I have more luck talking with a wall than I do with these folks. And so my usual response is to let them be right, express sorrow for the loss of what could have been in the conversation, and find a way to move on in the most loving way I possibly can. So it usually sounds something like this. "Ok. That sounds pretty final. In that case, There's really no point in us continuing on in this conversation. I'm sorry it turned out this way. Good Bye."