Another approach would be to record the receipt from the SKPaymentTransaction received on purchase or restore. On subsequent launches you could verify that receipt with the app store in the background, re-enable the ads for next launch if necessary.
Verifying Store Receipts documentation
Whilst I don't think this protects you from pasting in a valid user's receipt from elsewhere, it's more difficult to circumvent than toggling a boolean in the NSUserDefaults. Anyone going to this length is unlikely to pay for your app anyway.
