I have an in-app subscription product. When the app is loaded, I present a "subscribe now" screen to initiate the buy process. If the user cancels, the application MUST close/move to background/terminate since they have refused the service. What is the best method to use to do this?
This situation is addressed in the iOS Human Interface Guidelines, under "Don't Quit Programmatically":
Never quit an iOS application programmatically because people tend to interpret this as a crash. However, if external circumstances prevent your application from functioning as intended, you need to tell your users about the situation and explain what they can do about it. Depending on how severe the application malfunction is, you have two choices.
Display an attractive screen that describes the problem and suggests a correction. A screen provides feedback that reassures users that there’s nothing wrong with your application. It puts users in control, letting them decide whether they want to take corrective action and continue using your application or press the Home button and open a different application
For example, consider the iTunes Store or App Store apps, which are useless without a network connection. If you put your device in Airplane Mode and then launch either one, they will display a large Wi-Fi icon with a message saying an Internet connection is required. (I think an alert offers to disable Airplane Mode, but if you tap Cancel you'll see what I mean; the app will not quit.)
Be assured, if you simply force-quit the app, Apple will reject it.