I worked with a guy that said the moon landing was fake. The only evidence he could provide was the number of images of space that claim to be real but end up being doctored or fake, every time I called him out on his bs I was wrong and the image was fake. Thanks Scientists!
If it makes you feel any better, it's essentially impossible to convince a moon landing hoax believer they are wrong.
They effectively shine a light on the limits of each person's ability to justify their beliefs, vs. what beliefs are generally accepted just because nobody challenges them.
I've seen a similar effect in several Christian communities in the U.S. If a Christian is mostly surrounded by other Christians, it's easy to accept lots of beliefs because they're not challenged. But if they end up in an environment where people challenge those beliefs, or if there's a lot at stake for holding wrong beliefs, they tend to scrutinize their beliefs much more carefully. I think, overall, they benefit from pruning down their beliefs to a smaller set of better-justified ones.
So to the extent that flat-earthers help others think more critically, I'm glad they're part of public discourse.