Self cancel: https://helpx.adobe.com/stock/kb/cancel-membership-subscription-stock.html
If cancelling within the first 14 days of your subscription, you will get automatically refunded. Passed that date, you have an annual subscription of Adobe stock. Canellation will incure an early cancellationnfee of half of the remaining due.
If you haven't used the subscription, exceptionally, Adobe customer care may agree to cancel and refund without termination fee. But you need to contact Adobe customer care. If you have contacted customer care before on the same subject, use the case number to put the agent in the right context. There is no other way to get this done. But you may also try Twitter!
Adobe customer care can be contacted by beginning a secure chat session at https://helpx.adobe.com/contact.html?rghtup=autoOpen. Pop-up blockers need to be disabled! If the chat window fails to open, or is non-responsive, use a different device to start the interaction.
Support phone numbers may be found here: https://helpx.adobe.com/contact/phone.html. Please note that phone charges may apply.
For support via Twitter:
https://twitter.com/AdobeCare
(see also here: https://community.adobe.com/t5/creative-cloud-services/how-to-contact-adobe-support/td-p/11875703 or here
https://community.adobe.com/t5/account-payment-plan/how-to-contact-adobe-support/td-p/11843852)