After hanging up the call, dial the feature code *60.  When prompted, enter the 10 digit number you want blocked.  The system will repeat the number back to you and you must confirm it is correct otherwise it won't be saved to the list.  If a blocked number dials any of your phone numbers, the call won't ever reach any person (but will be logged) and the server will respond to it with a error code that means declined/rejected.