Facebook will ban groups that openly support QAnon, the baseless conspiracy theory that paints US President Donald Trump as a secret warrior against a supposed child-trafficking ring.
The social media firm said it will remove Facebook pages, groups and Instagram accounts for "representing QAnon" – even if they do not promote violence.
It said it will consider a variety of factors to decide if a group meets its criteria for a ban, including its name, the biography or "about" section of the page, and discussions within the page, group or Instagram account.
Mentions of QAnon in a group focused on a different subject will not necessarily lead to a ban, Facebook said, and administrators of banned groups will have their personal accounts disabled as well.
Facebook previously said it would stop promoting the group and its adherents, although it faltered with spotty enforcement.
It said it would only remove QAnon groups if they promote violence, but that is no longer the case.
The company said it started to enforce the policy on Tuesday but warned that it "will take time and will continue in the coming days and weeks".
Critics called it a much-needed, though belated, move by Facebook.
Jonathan Greenblatt, CEO of the Anti-Defamation League, said: "Now that they have announced that they will treat the QAnon ideology like the very real threat that it is, we hope that they will follow up with some modicum of evidence showing how the ban is being enforced and whether it is fully effective."
But the conspiracy theory has already seeped into mainstream politics, with several Republican running for Congress this year being QAnon-friendly.
In a blog post, Facebook said: "We've seen several issues that led to today's update.
"While we've removed QAnon content that celebrates and supports violence, we've seen other QAnon content tied to different forms of real world harm, including recent claims that the west coast wildfires were started by certain groups, which diverted attention of local officials from fighting the fires and protecting the public."
QAnon believes Mr Trump is on the front-line in a battle against enemies in the "deep state" and a child sex trafficking ring run by satanic paedophiles and cannibals.