I have met people who work in Silicon Valley who believe in a flat earth. After pressing the issue a bit, no matter what logical argument you throw at them, they can excuse it as "the earth might be curved a little bit, but it's not a sphere" (so no mathematical argument backed by facts they can see is going to persuade them) or "there's a conspiracy so no astronaut/satellite can see/show the true shape of the earth". It's not worth the time to engage someone who holds such a belief in conspiracy nonsense.