If you want to stop a user from contacting you, you can add their profile to your "blocked" list.
Desktop Version:
- Go to the user's page.
- Click "More" above the chat window.
- Select "Block User".
- Click "Block" to confirm.
Mobile Version:
- Go to the user's page.
- Find three dots to the right of the user's name and tap it.
- Select "Block User" from the drop-down menu.
- Press "Block" to confirm.