I could not find any documentation on Microsoft's websites regarding Ctrl + Space for deselection of items. However, on an unrelated website, https://winscp.net/eng/docs/ui_explorer_key, there is documentation regarding WinSCP, a free SFTP/FTP/SCP client for Windows. It uses some shortcuts that are also applicable to the Windows OS, such as Ctrl + Space for file selection/deselect. However Ctrl + Space does work, as stated above.
Anyway, the only different answer so far I could offer is:
Press key Delete to delete your file, then press Ctrl + Z to bring it back. Now you have unselected it.
User warning: it may have unintended consequences. Such as if you have configured files for permanent deletion and may not be brought back easily, as is the case in some external hard disk drives. Use this answer with caution.