iOS device users:
Press the home button two times quickly. You'll see small previews of your recently used apps. Swipe left on your device until you see Snoopy Pop, and then swipe your finger up on Snoopy Pop to close it. Press the home button again to remove the preview view.
Android device users:
Go to your settings and select the "App" or "App manager" option. At the top of your screen, select the "running" view and then scroll until you find Snoopy Pop. Select Snoopy Pop and then select "Force stop".