To Disable a shortcut in Ubuntu :
- go to Keyboard settings ,
- tab Shortcuts,
- search for the desired event ,
- select it and ...
- press Backspace
The text Disabled appears there.
This was needed when the shortcut for Evernote Clearly add-on in Firefox didn't worked: CTRL+ALT+Up was caught by Unity - it is assigned for Workspace Up. For me it is not needed - is enough a circular navigation with Workspace Right - ctrl+alt+right.