You could literally fly these people out into orbit in a spaceship, fly around the globe, and show them first hand...

...and they still wouldn't believe it.  They would say something like 'well, it looked flat to me, we were just
at different angles'... or ' must be some kind of trick'.

It really shows the power of belief. That is how strong the mind is, people
will believe the most ridiculous nonsense even in the face of overwhelming
evidence, even against their own direct observations.