Who Cares? is a common phrase used against people who think think people care about their subtle argument. Who Cares? is a common ploy of trolls trying to ignore or violate policy, when simply an administrator can simply ban them with the same response.
If someone is trying to pressure their pointless argument on you, a simple reply of Who Cares? will end the trolls moronic existence. If they are persistent ban them if you're an administrator or simply walk away if you're not.