How to cancel my Adobe Stock subscription without a fee?

You have taken the subscription in addition to your Creative Cloud subscription, by ticking the small button for the additional “30-day-free stock subscription”. What you did not read, was that it would convert into an annual subscription after the free month. Those conditions are with the subscription conditions that you said, you would accept, and they were sent to you with an e-mail confirming the subscription and the terms and conditions.

The early cancellation terms say that you need to pay half of the remaining due for not fulfilling your annual contract.

That's for the facts. The solution to this will be contacting Adobe customer support. If you haven't used the stock subscription, I suppose they let you negotiate favourable terms to ending this. You need to try. Good luck.

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, you need to accept cookies! If the chat window fails to open, or is non-responsive, use a different device and/or browser to start the interaction.

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)

Important: Adobe does NOT contact you unsolicited by e-mail or direct message. If you get contacted by direct message from a person, claiming to be an Adobe employee, look at that profile and check if they bear the “Adobe Employee” marking under their name. Adobe support does not use Skype to give you support.
If in doubt, ask the forum.

I'm always surprised to hear about Adobe subscribers who report that they didn't know they were getting charged for something for months. It's much easier to negotiate an early cancellation of an accidental, unwanted subscription if you catch it after the first month's charge to your credit card. Nevertheless, you will have to contact Adobe directly and explain your situation and your request. Cancelling can't be done here in this forum.. Adobe customer service can be contacted in a chat session at https://helpx.adobe.com/contact.html?rghtup=autoOpen.