Looking into App Store or Self Fulfilled but we don't know how to get the app service to stop working once they have it on their devices. It's a subscription model that we will charge $47 per month for the service, but if they cancel, we need to cancel their usage of it. Need to know how to launch this thing ASAP. Any help?
Without understanding what the App does, it's difficult to help. But one suggestion is to have the app, itself, require authentication on your server when used. The authentication could be as simple as them logging into the app for service. When they cancel their subscription, you suspend or delete their login information.
Another autentication method could be associated with their payment method. Keep a simple database or profile of this user (i.e. login and payment history as trial/active/inactive/suspended). I am assuming you already have their payment profile on file in your subscription model. On start-up, your app checks your database to see if this person's profile is in the active state, before it continues.
There are any number of ways to validate the user. One suggestion is to also offer a trial membership for 14 days. As part of the authentication process, you can check the date/time remaining in the trial period, send them reminders that their credit card will be charged at the end of the trial, etc.
When they have cancelled their subscription or their credit-card fails to process successfully - you update the authentication database appropriately. When they bring up the app service again and their status is inactive, the app outputs the appropriate instructions on how to refill or restart their subscription.
This is just a few ideas. I am confident that others will have better ideas.