From the admin panel, go to "pages" and there uncheck the "users" item : it will hide the page from your site.
But it unfortunately do not make it unaccessible : anonymous, i.e. non inscribed persons may access the users list by manually look for the URL as the follow : ./qa/users/
And also problematic is that it hide the page from all users, so they have to give in the url manually to access it...
It's not satisfactory but it's the only trick I have found so far.