I've started asking for a commitment up front whenever a disagreement comes up. So for flat Earthers, before bringing up any other points or discussing anything, just ask what would convince them to revise their views.

I sometimes preface this with saying that I think it's important to be honest about how certain I am about some position, and to state upfront what would prove me wrong. So I would be convinced that the Earth was flat if I ran an experiment, like a high flying balloon, that then showed 0 curvature in the Earth at high altitude where we'd expect some curvature. Then ask them the same question.

Now you're both on the same page, and if you're committed to resolving this question, you have a way to move forward and see who's right and you can do each step together to ensure no tampering, etc. And as a bonus, they get to learn exactly how science is done.