If you are referring to RedHat 8 (which is what I use), you can disable that option by going to control center -> system -> login manager

Then you switch to administrator mode which requires the root password and click on the tab that says Sessions. Here you can choose to only allow root to shutdown and reboot or (the default) to allow everyone.

I think that is written to the file /etc/X11/xdm/kdmrc but this is the gui route to do what you are looking for.

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dAggressor