Block And Unblock Someone From Facebook Messenger

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When you block someone, they’ll no longer be able to contact you (ex: send you messages, call you) on Messenger or on Facebook chat. However, you can always unblock a person whenever you decide to do so. 

To block messages from someone:
iPhone and iPad:
Open a conversation with the person you want to block
Tap their name at the top of the conversation
Scroll down and tap Block
Tap next to Block Messages

Android:
Open the conversation with the person you want to block
Tap , scroll down, and then tap Block
Tap Block all messages
How do I unblock someone in Messenger?

To unblock someone on Messenger:

iPhone and iPad:
From Home, tap your profile picture in the top left corner
Tap People and then tap Blocked
Tap the name of the person you want to unblock
Tap next to Block Messages to unblock them

Android:
From Home, tap your profile picture in the top right corner
Scroll down and tap People
Tap Blocked people
Tap Unblock next to the name of the person you want to unblock.

• Source: Learn Facebook Basics