The closest thing to "harm" I can think of with Flat Earthers is the idea of crank magnetism: a Flat Earther is very rarely just a Flat Earther. They often believe in all manner of conspiracies that are just as ridiculous and end up taking over their entire worldview. And we see exactly the same dynamics with many SJWS; it might start off as a legitimate grievance or focussing on one cause that the SJWs hover around, but soon they get pulled into a vortex.
That being said, only an idiot could seriously propose that the solution is censorship. Firstly, part of the whole pull of conspiracy theories is that the Establishment is covering this up. If governments or the modern East India Companies of Big Tech (at least it's not the government!) decide to censor it, the harm is twofold: firstly, the conspiracists have literal evidence that the Establishment is trying to shut them down. Secondly, it will - no doubt about it - lead further people down the rabbit hole; just as people can fall off the edge into SJW nonsense when on the edge of the hole (BLM protests spring to mind as recruiting sergeants), people can - while defending a conspiracist's undoubted right to speak - end up looking through the material; thinking some of it makes sense; and going on from there.
Moreover, the real harm comes from the censorship itself that will be a breeding ground for future conspiracy theorists to emerge out of the woodwork, and much more powerfully in an intellectually-stunted society; as Christopher Hitchens said, we always have to ask ourselves the question: "how do I know what I think I know, except that I've always been taught it and never heard anything else?" If we can't answer that question it should concern us.
