The "Back" Button on Mobiles will always put an Application into Sleep rather than Close.
This is behaviour from Android NOT the Application., and I'm not sure there is a way to override it.
Of course if the App is merely put into Sleep rather than Closed, opening a "New" Instance will simply Wake the App again.
I've often wondered why Mobile Developers don't (like Desktop) include a "Quit" or "Exit" Button within the App itself.
You can of course force the App to close by using the Square Button (Active App List) and hitting the "X" in the Top-Right Corner.