In KDE that can be changed in the control center.
System>system menu's?
I have Mandriva Free, so the menu can be a little different, but by running kcontrol from a terminal you will start the control center. Then go to Regional & Accessibility -> Keyboard shortcuts where you search for 'log out'. Change that to none or something else.
Then go to Input Actions also under Regional & Accessibility and make a new action (Keyboard shortcut -> Command/URL (simple)).
I haven't tried this myself, so it may or may not work, but hopefully it will.