There is a section called "Extranet".
When you click on "Extranet", you go to the root page of the extranet section. This page has a template that includes a new menu on the left side (that has all the Extranet pages).
Each of those entries on the left side menu are "sections". The extranet is a "microsite". When you block the Extranet from the section record, the microsite is not blocked, only the main landing page of it because it is the root page of the Extranet section.
To block all Extranet content, you must block each Extranet section and then select the groups that you want to allow through blocking.