Then I point out that our society evolves through ideas. If we all have the same ideas then we are like some bird that can only eat one type of plant seed. Fine as long as there are plenty of seeds, extinct if it doesn't work out.
Society is similar, we need a range of ideas and viewpoints so that we don't get trapped by the wrong ones.
That usually silences them, and gives them something useful to think about. Few of them realize that their entire philosophy depends on their ideas being right, and that it will and does fail miserably because nobody can make the right decisions and manage society. That's what free markets are for.
Liberalism is a jealous faith. As Bill Buckley once remarked, liberals are all for different points of view — until they learn that there are different points of view.