While not required, having two browsers (ie Firefox and Chrome) or two computers will make this test easier to perform.
1. As user 1 go the messages page (My profile -> Messages) click on "Manage contacts" and search for user 2.
2. Click the "block user" icon next to user 2's name.
3. As user 2 attempt to send a message to user 1. Make sure user 2 (the message sender) is not an admin. The following should be displayed. "This user has blocked you from sending messages to them"