That's the best way to deal with them I find. You can hit them with ancient astronomy techniques, or the elcliptic, or GPS or gravity but that requires a debate audience with a lukewarm IQ and Astronomy has been largely phased out of education. "Where is the edge?" "What stops the oceans from going over the edge?" "So then there is a barrier, show me." "Still exploring, I see, well okay, where is the frontier then?" "Oh Antarctica huh, what about the Western frontier?" Rather than debate the mountain of evidence for Astronomy at a fundamental level just demand they produce any of the would be obvious evidence to the contrary.