Man, a lot of comments on here are giving me reddit debate lord vibes. People talking about "the truth matters", but the way a lot of them are saying, it sounds like they just want to ego boost and dunk on/bully someone that they perceive as inferior; which I suppose could also be called "asserting intellectual superiority".
Chances are that any argument you use on them is something they've already heard, and the more you push and demean them the more defensive they will probably get, and the harder it will be to convince them. And even if you did manage to pressure and shame them into believing the earth is round, that won't suddenly make them good critical thinkers.