Suppose it’s a girl you were involved with on some level.

Do nothing: conveys indifference. Leaves the door open. But having you on her friends list gives her validation, and enables you to FB stalk her unless you have huge self-control or just don’t care about her.

Unfriend: Conveys butthurt. She can see that you did it if she cares enough to look for you , or if you have mutual friends. But it sends the message that you’re no longer her orbiter, and you can leave enough content visible for her to see how awesome your life is without her, although the intent might be so clear as to defeat the purpose.

Block: you simply cease to exist in her digital world. Goodbye, ho.