Por accidente se me compro la membresia mensual de adobe stock, ¿Como hago la devolucion?

Go to https://account.adobe.com/plans and go to manage the plan you want to cancel. Hit cancel. If you are in the first 14 days of your paid annual subscription, you will get an automatic refund. 

If that does not work for you, contact Adobe customer support. After answering the initial questions from the bot, type "agent" to get a life agent on the line.

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.

What was the reason for rejection? Was it even submitted, or do you just want a critique? That's not normally done here unless you have specific questions.