Delete key assigned to a keyboard shortcut and now doesn't work

Remove an unwanted keyboard shortcut

Open the dialogue in System -> Preferences -> Keyboard Shortcuts and select the entry you need to delete:

Once the entry is selected you can either enter a new shortcut, or in case you want to disable a shortcut for this action press Backspace (not Delete as this is a key that could be assigned as shortcut as well). You can close the window then.

Please reboot your system for disabling a shortcut to take effect

Another way is to go to the gconf-editor and edit the binding in /apps/metacity/global_keybindings/ or /desktop/gnome/keybindings keys.