Innocent or not, Mr. Dawson’s videos contain precisely the type of viral misinformation that YouTube now says it wants to limit. And its effort raises an uncomfortable question: What if stemming the tide of misinformation on YouTube means punishing some of the platform’s biggest stars?

A representative for Mr. Dawson did not respond to a request for comment. A YouTube spokeswoman, Andrea Faville, said: “We recently announced that we’ve started reducing recommendations of borderline content or videos that could misinform users in harmful ways. This is a gradual change and will get more and more accurate over time.”

Part of the problem for platforms like YouTube and Facebook — which has also pledged to clean up misinformation that could lead to real-world harm — is that the definition of “harmful” misinformation is circular. There is no inherent reason that a video questioning the official 9/11 narrative is more dangerous than a video asserting the existence of U.F.O.s or Bigfoot. A conspiracy theory is harmful if it results in harm — at which point it’s often too late for platforms to act.

Take, for example, Mr. Jones’s assertion that the mass shooting at Sandy Hook Elementary School in 2012 was a hoax perpetrated by gun control advocates. That theory, first dismissed as outrageous and loony, took on new gravity after Mr. Jones’s supporters began harassing the grieving parents of victims.

Or take Pizzagate, a right-wing conspiracy theory that alleged that Hillary Clinton and other Democrats were secretly running a child-sex ring. The theory, which was spread in a variety of videos on YouTube and other platforms, might have remained an internet oddity. But it became a menace when a believer showed up at a pizza restaurant in Washington, D.C., with an assault rifle, vowing to save the children he believed were locked in the basement.

To its credit, YouTube has taken some minor steps to curb misinformation. Last year, it began appending Wikipedia blurbs to videos espousing certain conspiracy theories, and changed the way it handles search results for breaking news stories so that reliable sources are given priority over opportunistic partisans. And last summer, it was among the many social networks to bar Mr. Jones and Infowars.

In a multipart Twitter thread this month, Guillaume Chaslot, a former YouTube software engineer, called the company’s decision to change its recommendation algorithm a “historic victory.”