Flat earth nonsense is not worthy of any response other than ridicule.
Ummm....I don't know...I mean it truly is nonsense but I think history has demonstrated that you can convince a lot of people to believe ridiculous nonsense when the intelligent people who know better choose to remain silent because they underestimate the level of ignorance in society and overestimate the intelligence of the masses. It's easy to lose touch with the reality of ignorance when you gain a lot of knowledge. It's easy to assume that everyone knows or should know the things you do. Those are dangerous assumptions because they foster the kind of silence that allows idiocracies to take over.
I don't think it should be looked at as being unworthy of response. It should be looked at as safeguarding the collective knowledge of reality from the pernicious inroads of pseudoscientific charlatans. Never underestimate the potency of stupidity. Never underestimate the credulity of the masses!