Disable cellular data before using app
Without jailbreaking, this currently isn't possible. You can provide feedback to Apple regarding this on the following page:


http://www.apple.com/feedback/iphone.html


If you're jailbroken, I refer you to a fantastic answer that I received to one of my prior questions, which whilst isn't quite a duplicate per the comments above, can answer the question.


Where are cellular data preferences stored in iOS?