In my experience, flat-earthers should be ignored. I recall one who started a discussion in the Amazon Forums. His goal wasn't to learn anything, but to insult all the participants with his cultivated ignorance and to lodge complaints against anyone who pointed out the fact that he was nothing more than an intellectual illiterate with a closed mind.

"The world is my country; all of humanity are my brethren; and to do good deeds is my religion." (Thomas Paine)