Unfortunately I've met people who think RT is more trustworthy than CNN. Not a lot but I've also had people send me links from there to prove some argument etc.

Still not sure if banning it is the right answer.

I wonder if a solution is to iframe these sites with a warning about disinformation at the top of every page.

Maybe browsers will eventually have fact checkers analyzing content and guiding the reader but still letting them read if they choose?