Forget all the stuff about it being visible from the moon. If it were flat, don’t you think people who lived near the edge would be charging a mint to come and look over the edge? They’d make a killing.
Only if the edge is accessible, some flat Earth models postulate a setup where the edge can’t be reached or it is infinite/wraps around like a video game and has no edge.
Sounds like a hyperbolic plane or a torus, neither of which are curved like the earth (as measured by Eratosthenes…)
How about the need for time zones?
If it were flat, cats would have pushed everything off by now. 
Obviously there is no proof that a real Flerfer would accept. Obstinate blind faith and insanity have no intersection w the world of objective reality and logic.
IOW … Forget it Jake P-Man, it’s Looney Town.
Maybe someone should sell trips to the edge of the earth. These days I’m not so sure no one would fall for it.
But blind faith is usually a financial windfall for its leaders. Remarkably, the flat earthers have failed to monetize the gullibility of their followers. They should definitely book expensive cruises on cheap watercraft to the edge of the world and sink them once out of sight of land. Money + evidence of folks sailing off the edge of the world. Win, win.
Where would flat-earthers get the money for those trips? Shout up to their moms from their basements?
You would think, right? But the fact that this doesn’t happen just highlights how deep the conspiracy goes, and how powerful the illuminati really are.
I would trust them to wash my car. Not sure if I would buy a box of cookies from them…
A prominent believer.
It is unfortunate that it isn’t flat. If it was we could just go to the edge and sweep all our environmental waste underneath.
Yeah that must be it. It curves around to a … sphere.
Wait. I’ve got it.
The earth can’t be flat because turtles don’t exist.
Gotcha.
They do. They suggest this is why there are international treaties protecting Antarctica and why it costs tens of thousands of dollars to go there. It’s really just The Man keeping you from figuring out it’s an impenetrable ice wall.
Also, they sometimes suggest only the livable surface of the Earth is flat. The underside doesn’t have to be. It could be flat like a coin, a sheer cliff wall like a mesa, or even rounded like a lidded bowl.
Absurd, but there you are. The answer to your OP is that people can’t sell trips to the edge because of the big, secret government cabal.
I am afraid that there is no amount of proof that will suffice. Flat earthers have come up with an idea that is not disprovable at all-there is no “What would it take for you to change your mind?” question that they will not directly ignore, no evidence they will not claim is fake, no direct testimony they will not call a lie.