When Mormons say something outrageous, simply ask them calmly:
"Don't you feel foolish?"
Of course, they are going to hotly deny it but then later when they are alone, that little voice in the back of their minds is going to start whispering to them. They may wonder if they are actually being foolish. You all know my favorite thing to do when provoked is simply laugh and say "Oh my gosh, I'm so embarrassed for you" but it only works when I'm really mad and it makes the other person mad back at me. I'd prefer to make people think or at least feel a bit off center.