Hi - Rather than describe the process from scratch, I'm going to be lazy and offer a link to a recent thread regarding this very question. Hope it helps.
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 .