1. Log in as admin to Plesk
2. Go to Users and create a new user with either "WebMaster" role or "Web Appications" as the role (both are ones that someone might think should have access to Installatron).
3. Log out and back in as that new user
There is no access to Installatron. If they have "Web Applications" as the role, there's no way to navigate to Installatron. (This may not be fixable). If they have "WebMaster" as the role, they can even see the button for Installatron, but when you click it, you'll get:
Error: Installatron can only be accessed by the admin login specified in `/usr/local/installatron/etc/settings.ini'. (Currently accessing as `client1').
I doubt there's a fix for the web applications role, but it'd be nice if the WebMaster role could have access