When you post a response, you are automatically subscribed to to a topic. You can go back to that topic and uncheck the box that says "Follow This Topic" at the top of the page, and you will no longer receive email notifications for that topic. Note that if you again respond, you'll need to make sure you do that same.
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 .