When the notification for the assignment pins our users, it can only be accepted by one person. They have three hours to complete the assignment before it goes back to the main queue for other users. So it is kind of a first come first serve system.
When you block a person, they can no longer invite you to a private message or post to your profile wall. Replies and comments they make will be collapsed/hidden by default. Finally, you'll never receive email notifications about content they create or likes they designate for your content.
Note: if you proceed, you will no longer be following .