I have avoided so many pointless arguments (or "debates") by leading with this question! I ask, "is there something I could say that would make you change your mind?" If the answer is no—if they can't tell me what will move them off their position—then I can say, "well let's not waste our time here, yeah?" and change the subject.

It's not perfect. But with otherwise-reasonable people, it's a nifty trick.