In the default graphical environment, users logging directly into the
system are greeted with a login screen that allows any user, known or
unknown, the ability the ability to shutdown or restart the system. This
functionality should be disabled by setting
disable-restart-buttons to true.
To disable, add or edit disable-restart-buttons to
/etc/dconf/db/gdm.d/00-security-settings. For example:
[org/gnome/login-screen]
disable-restart-buttons=true
Once the setting has been added, add a lock to
/etc/dconf/db/gdm.d/locks/00-security-settings-lock to prevent
user modification. For example:
/org/gnome/login-screen/disable-restart-buttons
After the settings have been set, run dconf update.