Meta is aware of a loophole that lets the message spam tsunami through & engineering is working on a fix.

In the meantime, this might help. Navigate to:

  • Business Suite
  • Inbox
  • Settings
  • Chat Plugin
  • Customize your Chat Plugin
  • Turn off Guest chat
A screenshot from Meta showing the Guest chat option enabled. From the setting details: Let people chat with your business without logging into Messenger while using the Chat Plugin on your website.