Hi Brian,
there is no super safe solution for it...
you can setup a Webserver like nginx or apache2 which blocks android and iOS request (maybe by using a filter on user agent) hit the Alfresco repository. This will help that nobody can get access with the standard apps. Of course all these information in http headers can be faked, so it might be possible for users to use another app or to modify the existing app (code is open source) ...
hope this helps!
jens