This is the root of the problem. The fact that his content is on YouTube does not imply that it will proliferate.
> How are we going to solve it
Critical thinking skills are apparently undervalued. This is a problem that is hopefully solvable by education and public debate. Even if not, the situation in which a small part of the population believes in David's ideas seems preferable to me than one in which a small group of (non democratically chosen) people decide what is acceptable content and what is not.
> It's implied in the concept of a marketplace of ideas that some ideas will have value over others.
I agree, but different people will have different definitions of value.
> And so it seems implicit to the concept of free speech that one should have the freedom to accept or reject an idea, or try to disabuse someone of an idea you deem fallacious.
I agree, but censoring someone goes much farther than to disabuse. Too far, in my opinion.