In the upper left hand corner, next to ACTIVITY, you will see a magnifying glass, a plus sign and a little gear. Those are the search, add new and settings.
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 .