Also offer reasonable alternatives to explain gravity, or whatever name the flat earthers want to give that force that keeps us grounded. At least the sphere has a center point, from which the entire surface is perpendicular. Thus, everything falls straight down. A flat earth keeps us grounded.... how? Where is this pull coming from? A bunch of parallel lines across the surface of the Earth? The sphere makes more sense as far as gravity pull is concerned.
Offer reasonable alternatives to explain night and day and how the division moves across the Earth. Flat Earthers explain it with a sun that circles above the Earth, lighting areas as it passes overhead. What forces keep the sun rotating in such a manner? Why are some areas dark if the sun is always above the surface of the Earth? Again, the sphere model explains night and day much better.
The Flat Earthers main beef is the water staying on a curved planet. A spinning ball. Saying that gravity holds the water against the surface just doesn't cut it. They use micro experiments to try and explain a macro phenomenon. I think that's flawed, but no one will ever convince Flat Earthers otherwise.
Here's the deal.
Round Earthers probably shouldn't ridicule Flat Earthers for their opinions. But the opposite holds true as well. Alpha saying he KNOWS the Earth is flat, while the rest of us can only THINK the Earth is round, is tantamount to ridicule and dismissal. This sets off the whole back-and-forth, enough of which we've had on plenty of other subjects. News flash: Not everyone that believes the Earth is round is a sheep, believing whatever they tell us. There is plenty of logical arguments for a round Earth.... just depends on the color of the glass through which you look at it.
Said this before and I'll say it again. Someone comes at you with a 9/11 conspiracy theory, and you believe they're being serious right away. So the argument begins right away. With Flat Earthers, it's so odd to the rest of us, that the initial reaction is one of... "You're joking, right?" When it becomes apparent they're not, that's when the insults start flying and everything goes downhill from there (and keeps on rolling until it disappears over the curvature of the Earth). We can hardly be blamed for the initial reactions.