I don\'t currently use any apps that have an in app subscription, so I am not sure what the UI looks like in iTunes when do an in app renewable subscription.
I assumed
Update (as this is fairly old by now): There is no API to cancel an auto-renewable subscription. Apple's documentation suggests that the app links to:
https://buy.itunes.apple.com/WebObjects/MZFinance.woa/wa/manageSubscriptions
"Opening this URL launches iTunes or iTunes Store, and then displays the Manage Subscription page."