The "unsaved Project" files are stored in the iClone Temp folder on your PC.If my memory is correct this folder is stored by default in this path:
C:\Users\Your User Name\AppData\Local\Reallusion
Since the AppData folder is hidden you need to make sure that you have "Show Hidden Folders" selected in the Explorer View settings. Locate the folder and delete all of the files stored in it, this will solve your problem.
If you are having space problems on your system drive then I would also suggest that you place the Temp folder on another drive on your system - you can set the location in the Preferences settings in iClone.