參考自：http://askubuntu.com/questions/315625/how-to-disable-the-shortcut-ctrl-alt-arrow-in-gnome-3-8
The new keybinding you can define in 'Settings-->Keyboard-->Shortcuts-->Navigation' is appended to the previous/default one.
for example:
gsettings get org.gnome.desktop.wm.keybindings switch-to-workspace-down
['<Primary><Shift>Down', '<Control><Alt>Down']
I managed to remove the default keybinding using the following commands
gsettings set org.gnome.desktop.wm.keybindings switch-to-workspace-down "['']"
gsettings set org.gnome.desktop.wm.keybindings switch-to-workspace-up "['']"
This clears all defined shortcuts for those tasks. And then just use 'Settings-->Keyboard-->Shortcuts-->Navigation' to define the desired shortcuts.
