Not that this difficulty means we shouldn't try.
Not exactly the best way to convince people that restricting speech is actually good.
Most people agree that shouting fire in a crowded theatre is the sort of speech which should be restricted, wherever the argument came from.
All the more reason to not silence cranks and conspiracy theorists.
From John Stuart Mill's On Liberty:
> But the peculiar evil of silencing the expression of an opinion is, that it is robbing the human race; posterity as well as the existing generation; those who dissent from the opinion, still more than those who hold it. If the opinion is right, they are deprived of the opportunity of exchanging error for truth: if wrong, they lose, what is almost as great a benefit, the clearer perception and livelier impression of truth, produced by its collision with error.
– John Stuart Mill, On Liberty https://www.utilitarianism.com/ol/two.html
But still, that's arguably distinguishable from creating a panic by shouting "fire" in a crowded theater that's undoubtedly not burning. In that crowded theater, it's the stampede that gets people killed in the moment. I do see the analogy to Internet meme explosions. But connections between actions and consequences are weaker and slower enough.
> A poll of Americans in 2013 by Public Policy Polling indicated that 4% of registered voters (±2.8%) believed in David Icke's ideas.
But at least some of them were joking, right?
0) https://en.wikipedia.org/wiki/Reptilian_conspiracy_theory
Yes, we really can't have it both ways. Either we have free speech for everyone, or we have censorship. There are exceptions, of course: calls to violence, libel against non-public people, and so on. And yes, also inciting panic by crying "fire" falsely in public places. But I believe that's distinguishable from spreading "fake news".
The problem with censorship is needing to trust censors. And even if we have honest censors, how do we know what the "truth" is? Do we trust "authorities"? That seems iffy, given how often we've learned that they've been dishonest or just plain wrong.
Also, authoritarians always start with the edge cases. And then they widen the scope. By the time your position -- which always seemed so reasonable -- becomes unacceptable, it's too late.