It was a sporadic problem - sometimes OK, sometimes not. However, I've logged a support request but, touch wood, it's been OK for the last couple of days.
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 .