If your desktop becomes cluttered with open app windows, you can use Spaces to organize the windows into groups. Some Apps create a new Space automatically when you switch to full-screen view.
The trouble is, sometimes you cannot delete or remove these Spaces, even when the App has stopped running.
It took me a good moment to find a solution for this, as the “X” to delete that was supposed to appear over a Space when opening Mission Control would never work. Even pressing “Alt / Option” in Mission Control to get the “X” to appear for all Spaces didn’t work.
The trick is to close all open Apps, delete the Spaces preferences file, and then reboot before the file is automatically regenerated.
After a reboot the undead Space will no longer be there.