In the Keyboard Shortcuts preference pane, this is the one shortcut that can't be disabled. It's disabled if you turn off Chinese keyboard input, but I want to keep the keyboard input.
I got hopeful when I found this technique for editing the symbolic hotkeys plist:
https://stackoverflow.com/questions/15719135/how-to-disable-keyboard-shortcuts-in-mac-os-x
But I haven't been able to find a symbol corresponding to this shortcut.