Sorry to hear that you are facing issues while cancelling your Stock subscription. Since you are unable to self-cancel your subscription, I would request you to contact our support team Support FAQ: How can I contact Adobe for support? for the cancellation requests.
If you are unable to reach our support team through the above-mentioned link, please see Contact Support FAQ: Unable to reach support through the "Contact US" link
Feel free to update this thread if have any additional questions.