I'm more wondering about when would one need any rebuttal to start with. Most online comments are probably trolls, and if not, it's not worth your time anyway. Is it common to have friends or relatives with such a feeble grasp of reality?
I told her once that Alex Jones certainly has enough money to rent a jet to fly into a chemtrail and take some samples or take a ship and go straight and see what happens. But of course there tons of government conspiracies to prevent these things. In the end these people don't really want to know. They just have a deep discomfort with the current state of things.
This is a very succinct description of the mindset of the flat earthers I have talked with.
The outcome was mostly disappointment for me. They don't have anything like a reflectively consistent worldview. They don't in general have positive statements (other than that the earth is flat). Their beliefs are almost entirely negative, and phrased as arrogant dismissal of mainstream belief and evidence.