Hi Stephane,
Just to clarify, when I say accessible via direct link… authentication is still required (an unauthenticated user wouldn't be able to view the page). Also, the user would still be restricted by their usual permissions - so in most cases they'd just be be able to read repository information (apart from nodes they do not have access to, e.g. private sites, etc). If you want to completely prevent a user from accessing the page (even via typing a URL into the browser address bar) then you'll need to modify the webapps/share/WEB-INF/classes/alfresco/site-data/pages/repository.xml to change the <authentication> setting from user to admin (this will restrict anyone without admin privileges).
Hope that helps,
Regards,
Dave