of course it may be possible to hide the global addressbook via some markup or stylesheet modifications, BUT i don't think it's what you want. Most use-cases for hiding the global addressbook is to hide other users on the system. This will be very complex and faulty because the GUI uses entries at the global addressbook allover for different functionality. Even if you hide the folder, the entries of the global addressbook is being sent to the UI. A user who analyzes the websites communication will notice all the data of the global addressbook entries.
We already had this feature requiest iirc, but unfortunately there is no ETA when this is going to be implemented.