Message requests and blocking
Who can message you, and how to stop someone.
A first message from someone you don't follow arrives as a request. It sits in the Requests tab until you accept or decline, and declining isn't shown to the sender.
On a public profile you can turn off requests from people you don't follow entirely, in Settings under Notifications.
Blocking someone from the conversation menu stops messages and requests both ways and stops them following you or commenting on your posts. They aren't told.
Didn't answer your question? Contact support.