If you're mentioning "CTL+ALT+DEL" I guess you're using Windows, so you can click on the standalone app window (to make it the active one) and press ALT+F4 on your computer keyboard... or press ALT+Spacebar to get the window context menu on the top left corner and choose the "Close" option.
The issue might be the screen height which isn't enough to fully show Spark's window and that hides the usual windows top buttons bar where you expect to see the close button.
