I have a non root user in my ubuntu desktops the user can change the time manually from Gnome. I have installed a NTP Server in my environment. Now, I don't want this user to be able to set the time manually. What I exactly want is to disable the "manual" option from date and time settings on All Gnome date and time settings windows.
org.freedesktop.timedate1.set-time
would be the one. You could adapt askubuntu.com/a/582233/158442 to use this policy.