The trouble is that they reject most maths and science, part of their argument is that the educational institutes are filled with liars. Which makes it difficult to formulate a scientific argument against them.
In order to engage in rational debate with a person, both parties must agree (implicitly or otherwise) on
1) the sorts of arguments that are permitted, and
2) the sorts of premises that are accepted.
Flat-Earthers and Round-Earthers on this forum agree on neither of these points, primarily (2). FE'ers insist that only what one sees with one's own eyes, or derives from first principles, are acceptable as premises. They then go on to make statements about things they have never seen themselves and have not derived from any axioms; their willingness to act in a contradictory manner is the most fundamentally asinine way in which they disagree with RE'ers on point (1). Furthermore, they reject all established science and mathematics (another disgareements on (1)), but don't hesitate to butcher science and mathematics if it suits them. Lastly, FE'ers see fit to quote scripture to support their claims, even when it doesn't support their claims, which is of course logically void on at least two counts.
Conclusion: no rational discussion can be had between FE'ers and RE'ers. What sorts of discussion are we left with? Sophistry, is an option, but there is no audience. It seems we are left with the option of crushing the egos of flat Earthers and making them cry.
Nevertheless, if anybody's on the fence, I will continue to offer constructive, logical arguments and analysis. Alternatively, if a person argues for RE in an idiotic way, I will point out the flaws. So this forum can still be of service to people interested in rational thought.
-Erasmus