I think there's a really important distinction between ideas that are simply wrong like phlogiston or perpetual motion, and things like antisemitic/white supremacist conspiracy theories (if you look into this subject you'll find that they're one in the same, with antisemitism being the foundation of white supremacist conspiracy theories) that have evolved to spread virally though a population and perpetuate bigotry and motivate violence. It's a pretty literal form of "mind virus," and they're subject to selective pressures that cause them to literally evolve to be very difficult to discard once you've accepted them. Phlogiston isn't a closed system of ideas that absorbs all criticism of it and inverts it into evidence that "they" don't want you to know about the ideas.

Consider flat earth conspiracy theories (which are not nearly as harmful as white supremacist ones and I don't mean to equate them, only to pivot to a different example that's a less touchy subject); it's trivial to debunk FE arguments, and people have, exhaustively. But unlike phlogiston, that didn't make the FE stop having currency in our discourse. It's not just something wrong which will lose out in the marketplace of ideas. In the documentary "Behind the Curve," several of the flat earthers come up with really compelling experiments that debunk the flat earth. But they aren't actually convinced by their own results. A conspiracy theory or system of conspiracy theories survives by being an endless font of explanations that paper over the inconsistencies. Those that are unable to do this either die off or are incorporated into a larger system of theories that have more explanatory power.

As a matter of free speech it shouldn't be illegal to hold or to discuss these views, but unlike ideas that are simply wrong they create and enhance hazards as they spread. They isolate people within these communities; I don't know if you've ever spoken to someone who is a hardcore conspiracy theorist, but it can be really difficult to connect and interact with them because they will have really strong responses to seemingly innocuous things, and constantly insist that you're a rube for not thinking along the same lines.
This often erodes their relationships with people outside the culture of the conspiracy theory (which are often very loving & supportive places). These communities form a millieu which hate groups and fascist movements can be drawn from, not because these individual people are especially hateful, but because they've been indoctrinated into a cult-like setting where a proposition like, "there are children being held in the basement of this particular pizza parlor, someone needs to step up and save them" is taken quite seriously. You don't have to be a hateful person to show up to a pizza parlor with a gun (or commit other acts of violence) once you've developed the sincere belief that children's lives are at stake and that it's your personal responsibility to defend them.

How to properly counter this phenomenon is largely an unsolved problem, and I'm not suggesting that a dictatorial approach is warranted (that'd pretty obviously backfire by seemingly validating the conspiracy), but there's more nuance here than orthodoxy vs. free exchange of ideas. We need to find a way to continue the free exchange of ideas while addressing these dangerous, emergent phenomenon.

I'm currently working on software to help people create a personal epistemology, to compare theirs with others, and to collaborate on a shared epistemology. There's various reasons I think this is a good idea, but one of them is that I think being able to demonstrate what you believe and why would be beneficial to our discourse and hopefully counteract conspiracism.