So the way to call their bluff and expose their relativism as purely tactical is to insult the moral values they cherish. For example, you could say, “I don’t know why we have laws outlawing racial discrimination and gay-bashing. How can people presume to legislate morality?” Or “I am surprised people object so strongly to the Confederate flag. I don’t have a strong view one way or the other, but since morality is relative, can anyone really say that the South’s cause was wrong?” Or how about “What’s the big deal about the environment? Why should I preserve the planet for the sake of future generations? What have future generations ever done for me?” Say these things as if you believe them, even if you don’t.