You have two ways to block a user
From Your Messages
Click the chat icon in the header to open your messages.
Find and select the conversation with the person you want to block.
Click the three dots next to their name in the message thread.
Select "Block".
From the User’s Profile
Go to the user’s profile page.
Click the three dots next to their name.
Select "Block."
Once blocked, the user won’t be able to interact with your content, view your profile, find you, or send new messages. However, if you had an existing chat with them, they will not be notified that they've been blocked. They might still try to send messages, but you won’t receive and see them.
If you change your mind, you can unblock users anytime in Chats by clicking "Unblock," or through "Settings" under the "Security and Privacy" section.
More information about blocked users can be found here.