To make clear why do we have "Flat Earth" people and why I think we should leave them alone.
They basically don't have trust in Science and now they're kind of "Protesting" against Science, I don't know why specifically for "Flat Planet" but, they it seems like they have never knew that the Earth is Spherical which means that they're not even pretending as if it is Flat, but rather when they do their own research to prove themselves - they usually prove themselves wrong. And they become normal again.
So might not be worth spending time with them because in the end they will get it by themselves.