I use GNOME Classic. There is no Ctrl+Alt+Left/Right shortcut in System Settings -> Keyboard -> Shortcuts.
It should be configured somewhere else. But where?
Remove the default keybinding using the following commands
gsettings set org.gnome.desktop.wm.keybindings switch-to-workspace-left "['']" gsettings set org.gnome.desktop.wm.keybindings switch-to-workspace-right "['']"
In my system, those configs are available in System Settings -> Keyboard. Please double-check.
If those aren't available, you can try add them with
gconf-editor. If you use Metacity, simply go to
/apps/metacity/global_keybindings and add the corresponding values:
If you use Compiz, you can follow the steps provided by @ams
CompizConfig Setting Manager (it's not installed by default though).
Then go to "Desktop Wall Plugin", and it's on the "Bindings" tab.
That's assuming you are using Unity (full 3D), or (edit) Gnome with effects enabled.
Go to /apps/metacity/global keybindings and change the values according to my screenshot: