I would just laugh. I don't think anyone today really believes the earth is flat. If someone told me they believed that, i'd have a hard time taking them seriously. I'd think they were just playing with me. I think a lot of people do that. It's kind of fashionable today, in some circles, to espouse crazy ideas, like that 3 million illegal aliens voted in the 2016 election, or that illegal immigration is the worst problem the US faces today. Or that Putin is a great leader. Or that Trump is a genius. I think the people who say these things know they're wrong, but we live in a Post Truth culture!
I had a roomy once who believed that math was arbitrary, that the only reason one and one equals two is because we all agree it does. "What about when two raindrops come together? They form ONE raindrop, right?" He also believed that advertising should be illegal because it's all lies. And this was a guy who was pulling straight A's, working on a Master's in Social Sciences. He was a tournament level chess player! And the first thing he did in the morning was to smoke a joint. (I don't think I ever saw him straight!) I have to admit, it was hard to be friends with him.