You FErs using the same tactic that a fanatic does when they he/she completely cornered. Nobody can give you absolute undeniable proof for anything. Ever. If I want to, I can say you don't exist. You could sill come to my house, you could talk to me punch me in the face, whatever, it doesn't mean I have to believe you exist. Dare I say it!? it's a conspiracy!
We cannot prove the sun is a star.
We cannot prove gravity is a known force in the universe.
We cannot prove sub atomic particles exist.
We cannot prove the existence of virtual particles.
We cannot prove the existence of other planets.
And we sure as hell cannot prove that the earth is round,
we can try, we can use logic and reason and make educated guess and factor out equations.
But whats the point? If they have a completely different belief system then we really have no ground to push off of. End of story.
We cannot win this battle, because everything we say or do will be denied and explained by some outlandish theory that doesn't make sense or has no solid evidence.