Allow WP permissions to user ID XX which in turn removes it from everyone else

I came accross some code to add to functions.php where it granted permissions to a certain user ID, and everyone else it was removed. I lost that link and cannot find it. Does anyone know what I am talking about and how to do it? Seems like the best way to lock down your site when you have many Authors, Editors and other Admins.

Thanks in advance.