I teach at a college, and last year, I had a student who wanted to write his argumentative essay on why the earth is flat. I took the opportunity to discuss with the class the foundation of a sound argument, and why a logos (logical appeal) is the most important part in the defense of a thesis. In order to obtain a solid logos, one must use verifiable evidence from legitimate sources.
It was at this point that I told the student that he was welcome to construct his argument, but only if he could find reputable sources from the scientific community who could back up his claim, and not simply the say-so of a bunch of bloggers and Youtubers.
One week later, he dropped his argument and switched his topic to government spending.
EDITED TO ADD: I have no idea if the student actually believed the earth is flat or whether he just thought it'd be interesting to argue its point. I didn't discourage his choice of subject, but simply pointed out that he would have to defend his thesis like any of my other students, using all 4 appeals and 5 reliable, reputable sources.