Arguing with flat earthers (or creationists, and frequently they're the same people) is a waste of time - they have soundproof heads. But good luck with your endeavor. :-)

Me, I have seen the curvature of the earth for myself. It's easy enough if you have ever been out to sea on a boat: within a few kilometers, you can see tall buildings on the shore "sink" below the horizon. And this is always my answer to them: I have seen it myself, and observation always trumps any amount of theory or armchair philosophy.

I am nowadays more interested in the psychology behind such beliefs. I think that very often, it is not a matter of education. These people have some sort of deep-seated need to believe what they believe. Maybe they are longing for simpler times, or maybe it's something else. But you will not convince them even if you take them for a ride on a spaceship.

It is a good thing though to show all the more or less neutral observers how nutty their ideas are, so do keep up the good work! :-)