You can choose which domain you want to use as your default. So even if you register other domains, they will not be set as your default until you explicitly set them as such.
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 .