Flat-earthers are generally used as a teaching opportunity rather than seen as actual serious candidates for debate. The question of how we know the earth is spherical can present an interesting opportunity to teach some science, and the idea that the Earth is flat can be a good example for learning about denialism and how critical thinking can go wrong. But these things are only useful to people who value rationality and evidence. Flat-earthers generally fall into three camps, none of which value accuracy.
Some accept a flat Earth due to religious reasons, and that is the extent of their arguments. A debate about the Earth with these people would be a red herring. The real issue is faith and doctrine, which is a different debate entirely.
Some accept a flat Earth because of a Maverick factor. It's essentially about dissent and conspiracy mongering. These are the types who think they have overturned Newton’s laws and relativity. A debate with these people will, at best, devolve into a debate about the scientific method. You'll find they simply disagree on fundamental concepts like quality of evidence, consistent logic, and basic epistemology.
The third variety tends to be nothing more than trolls who think it's funny to get people worked up over something as stupid as the idea that the Earth is flat. Obviously a debate with people like that has no hope of being productive.
Many science communicators who take the time to debate subjects like Bigfoot and crop circles are often told they are wasting their time. It's often said that there are more important and pressing topics like medical fraud and climate change to be focused on. Despite being constantly told that, they press on because they do see a value in engaging people and promoting critical thinking, even on silly topics. But, even those people do not bother with flat-earthers. It's truly the bottom of the intellectual barrel, the very silt which remains after every other crank filter which usually catches stupid has failed. It's the place where even chemtrail truthers and homeopaths point to and laugh. If this is the place someone wants to be, let them. There is nothing you can do to get them out.
