Oh, I think there are people who really believe this. I have a FB friend who argues with flat-earth friends of his all the time when they post things to him.
They believe all of the photographic evidence is faked. And like another poster said, they try to explain one thing away with reason A and another phenomena away with reason B and another with reason C. But reasons A, B, and C can't all be possible. It's nuts. Like so many conspiracy theories are.
And unlike deriding the evidence, they can simply go get evidence to support their claim. You don't need to be a billionaire to get a boat, get some drone equipment, sail out to this edge of the world and use your drone to fly over the edge and get some pictures.
Instead of complex mental gymnastics to prove their theories, go get some evidence. Oh, you can't.