You can't win an argument with a Flat Earther, folks. If I hold a mirror up to your face, you will see that your head is round, with peaks and valleys and curves and depth. The Flat Earther, however, will examine the mirror, show you that it's flat, and then proceed to tell you that the roundness of their head is a hoax and you should never believe what mirrors tell you. Their capability to deny what they see with their own eyes exempts them from all scientific, logic-based explanations. Just give them their billboard (and tax exempt status) and they'll be on their way.