If you purchased an Adobe Spark subscription using an Apple device (iPhone or iPad), Apple requires that you cancel your subscription through the Apple App Store on your Apple device (or through iTunes on your computer).
To cancel your subscription, first turn off the automatic renewal of your subscription through the Apple App Store or iTunes. Once automatic subscription renewal is turned off you will not be billed for future renewals, but you will continue to have access to your existing subscription until it expires.
For additional details on how to cancel your iOS subscription, please review Apple's documentation.
If you have any questions or need help canceling your subscription and turning off auto-renewal from the App Store or iTunes, contact Apple Customer Support.