If you have been receiving threatening, harassing or annoying messages from a specific Facebook user, prevent that person from sending you any more messages by blocking her profile. Alternatively, if you regularly receive messages from strangers in your inbox, consider editing your Facebook message filtering settings.
Block a Profile
Blocking a profile unfriends that person and prevents her from interacting with you by sending you private messages or sharing posts on your Timeline.
Mobile App
Visit the profile of the user you want to block and tap the three-dots icon to open a pop-up menu.
Touch Block to block that user.
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Facebook Website
Visit the profile of the person you wish to block and open the three-dots menu.
Select Block from the menu.
Click the Confirm button to finish blocking the selected user.
Edit Your Message Filtering Settings
Much like the spam filter Web mail platforms provide to their users, Facebook comes with a message filtering feature that automatically directs potentially unwanted messages to a special folder called Other. While that feature's default setting does not filter messages from people you may know -- such as users who are friends with your friends -- the Strict Filtering setting lets you keep your inbox free from messages sent by users you did not friend.
Mobile App
Tap the triple bar icon to open the app's menu and select Privacy Shortcuts.
Select Strict Filtering.
Facebook Website
Click the padlock icon and select Who Can Contact Me?
Select Strict Filtering.
