You have control over YOUR site, so you can change the theme on your blog, your wall, etc. But the threads belong to the community, and the themes are pre-set.
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 .