There are two access rules which govern access to the Untangle itself, which can be found by navigating to Config > Network > Advanced > the Access Rules tab.
By default the #2 & #3 rules are created by the system. This will either allow or reject access to the admin login page via HTTPS. The second rule is for granting access through HTTPS from any WAN connection. To stop users from accessing the login page on your device from outside of your network, you will need to have this rule disabled.