Comment # 4 on bug 907907 from Wolfgang Bauer

(In reply to Alberto Passalacqua from comment #3)
> I also tried to change the DISPLAYMANAGER_SHUTDOWN option to "root" in
> /etc/sysconfig Editor. However, any user can still shut the system down.
> 
> Thank you for your help!

AFAIK this setting is not respected any more because it confused users, see
https://bugzilla.opensuse.org/show_bug.cgi?id=860507#c8 ...

Try to configure this in KDE's "Configure Desktop" (aka systemsettings)->Login
Screen->Shutdown.

But be aware that you probably won't be able to enter root's password because
of  bug#772344.