I'm not sure that I have ever run into someone who believes that the earth is flat, but they do walk among us. The best strategy to debate a flat-Earther is to walk away and don't engage, but if you do get sucked into that black hole, this article offers some good advice on how to counter their arguments. 

Flat-Earthers don't arrive at their beliefs through scientific observation. Instead, they suspect they are being misled and lied to. The real question isn't "why do people believe the Earth is flat?" But rather "Why do some people believe in a conspiracy" The answer is simple: a lack of trust.

Many people don't trust the representatives of society and that distrust extends to scientists, academics, and government agencies. By claiming that Earth is flat, believers are expressing a deeper skepticism about institutions and science itself.

So if you find yourself talking to a Flat-Earther, skip the evidence and arguments first. Instead, ask yourself how you can build trust. And then you can counter some of their claims with this information:

Claim: "NASA fakes space images."

Response: Satellite data from multiple independent space agencies, including NASA, ESA and private companies, confirm Earth's roundness. Amateur astronomers and even high-altitude balloons show the same curvature.

Claim: "The horizon always looks flat"

Response: The horizon appears flat because of human perspective and scale. However, from higher altitudes — such as those captured by weather balloons or aircraft — the curvature is clearly visible.

Did a Flat Earther accidentally prove Earth is round?

Yes! In 2018, a group of Flat Earthers conducted a $20,000 gyroscope experiment expecting to prove no rotation. Instead, it recorded Earth's 15-degree-per-hour spin— confirming our planet's roundness.

That last one is my favorite! But there are plenty more in the article, and other information such as why people still believe the earth is flat.