I'm going out on a limb, but I'll take a stab at AR 25-50. I found all the proper ways to address written correspondence, and right at the front of the AR it says you should use face to face meetings or telephone calls for correspondence when possible. I know it's in there somewhere, but I don't have time to hunt right now.
Cubicle Boy is right, AR 600-20 chapter 1-6 Military grade and rank, table 1-1 states what the form and address of each rank is. However there is no specification for face-to-face or phone. Best to stay formal.
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 .