If you want the app to completely stop and not generate any Sweatcoins then you can force quit it (when you double tap on the home button and flick the app away).
If you want to stop the app for a long period you can go to your device Settings > Privacy > Location Services > Sweatcoin > Switch Off (or something similar depending on your device).
Don’t forget to turn it back on when you want it to start generating Sweatcoins again. Needless to say that in all of these scenarios no Sweatcoins will be generated...