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 Safari Smash, and then swipe your finger up on Safari Smash to close it. Press the home button again to remove the preview view.
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 Safari Smash. Select Safari Smash and then select "Force stop".