In Dinesh D'Souza's book What's so great about Christianity, D'Souza says in Chapter 20: Natural Law and Divine Law: The Objective of Foundations of Morality:
"...What, then, are we to make of relativism—the influential doctrine that says that morality is relative? I agree that relativism has something going for it, in that people even within our own society disagree about the content of morality. There are also debates about the priority of one moral principle over another. Different individuals and even societies disagree over how a moral principle should be applied in a given situation. But on the existence of moral standards there is no disagreement. Consequently relativism of moral belief and practice in no way invalidates the claim that morality is absolute. Indeed I submit that not only is morality absolute, but everyone, including self-proclaimed relativists, knows that it is absolute. Relativism in its pure sense simply does not exist.
If you are confronted by a relativist who insists that all morality is relative, go ahead and punch him in the face. If he does not respond, punch him again. At some point he will protest, "That's not right. You shouldn't have done that" Then you can explain to him that your actions were purely educational. You were simply demonstrating to him that even he does not believe his relativist doctrine. His objection was not "I don't like being punched" but rather "you should not have done it" He was appealing to an unwavering standard, which he expected you to share, that what you did was wrong.
Another way to make the point, when you hear people solemnize about the relativism of values, is to find a value they cherish and excoriate it. This is a useful approach because most of the time, when people deny absolute morality, they are engaging in a rhetorical strategy in order to undermine some particular moral belief you hold and they don't. Social liberals, for example, often discuss topics like drugs, pornography, and prostitution by saying, "How can you impose your beliefs on me? Who is to say what's right?" They seem to be denying absolute morality. If they are not self-aware, they might even believe this.
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.
Before you are finished, I think you will find your relativist up in arms, insisting that prejudice and racism are immoral and unjust, and that we ought to have laws restricting them and protecting the environment. The person who affirms these doctrines is not saying that his views on bigotry and environmentalism are simply a matter of personal preference. He is implying that everyone should feel this way, and no decent person should behave in a manner contrary to his principles. He may ignore the moral law in the way he acts toward you, but he is quick to invoke it as a standard for how he expects you to act toward him. In short, his actions confess that despite his loud denials, he, too, espouses morality as an absolute..."