How to unbind Command-Control-Space key from Mac OS X 10.9?
You can create custom keyboard shortcuts for most app's menubar choices in System Preferences. If a desired key combination is losing precedence to a default shortcut that you don't use and can't easily disable, simply override it with a new, unobtrusive shortcut.
Open System Prefs / Keyboard / Shortcuts. Select App Shortcuts from the left pane. Toggle the All Applications category's triangle in the main window to point downward (if it's not open already).
If there's an item named Emoji & Symbols* shown there, then click its shortcut combination and enter a new shortcut (such as option-shift-command-t, in this case).
If there's not an item named Emoji & Symbols under All Applications, click the + button at the bottom, type or copy-paste Emoji & Symbols, and then enter a new keyboard shortcut (option-shift-command-t, or anything really). This will free the control-command-space combination for you to use as a specialized shortcut elsewhere.
To remove your custom shortcut, just click to highlight it in the main window of this preference pane, and click the – button at the bottom. The custom shortcut will disappear and the default action will resume.
*Note: On versions older than Mac OS 10.10.3, the menu item is called Special Characters… instead of Emoji & Symbols.
At least on macOS Sierra to macOS Big Sur ⌃Space is the default binding for Select the previous input source which is on by default (even if only one input source is activated).
You can free it by:
- Open System Preferences
- Go to Keyboard > Shortcuts > Input Sources
- Untick "Select the previous input source"
Afterwards, you should be able to bind it as expected.