if i understand correctly, when you say "Ever since then, whenever I open Screenshot Captor a small window pops up that says License Key Saved", what you mean is every time you restore it from the tray.
that window should have an OK box to close it, try clicking on the window and hitting Enter.
in the worst case, until i fix the display of that box, you should still be able to KILL screenshot captor from running (hit ctrl+alt+delete and find ScreenshotCaptor.exe and terminate it). When you restart the program you shouldn't see the window anymore.