I am trying to figure out how I can disable ctrl-alt-backspace and ctrl-alt-delete from doing anything. In the good ol’ days one could add some entries to the /etc/inittab file but is there a way to disable these key combinations so a user is unable to reboot or restart the system using these keys with systemd?
Not that this should matter but I am running XFCE with a clean DVD install of openSUSE 13.2.
Well, I doubt that you could ever disable Ctrl+Alt+Backspace via /etc/inittab, as this is a key combination to which Xorg reacts. 
You can disable it in Xorg’s config, e.g. create a file /etc/X11/xorg.conf.d/10-dontzap.conf (or similar, the exact name doesn’t matter at all) with this content:
Section "ServerFlags"
Option "DontZap" "true"
EndSection
See also “man xorg.conf”.
When you press Ctrl+Alt+Del, systemd just changes to Ctrl-Alt-Del.target.
From “man systemd.special”:
ctrl-alt-del.target systemd starts this target whenever Control+Alt+Del is pressed on the console. Usually this should be aliased (symlinked) to reboot.target.
Change this symlink (/usr/lib/systemd/system/ctrl-alt-del.target) to whatever you prefer (to disable it completely, link to /dev/null).
It’s preferable to create your override symlink in /etc/systemd/system though.
sudo ln -s /dev/null /etc/systemd/system/ctrl-alt-del.target
Not that this should matter but I am running XFCE with a clean DVD install of openSUSE 13.2.
It does matter, as the desktop can handle Ctrl+Alt+Del itself. KDE does e.g. (it shows its Logout/Shutdown/Restart dialog then), no idea about XFCE.
In this case you would have to configure it in your desktop’s settings.
Thanks Wolfi323 that worked like a charm!