You will have to get back into the old account to stop the subscription.
● If a message says "your account has been disabled in the App Store and iTunes Store" or "You cannot login because your account has been locked", then you must contact Apple; you cannot resolve this yourself. Apple provides a link for this specific issue. Tap or click here ➞ https://getsupport.apple.com/?caller=kbase&PGF=PGF63005&category_id=SC0245&symptom_id=23362 .
● If your account has been disabled for security reasons the solution is to reset your password as outlined in this document: If you forgot your Apple ID password - Apple Support
If you find you can’t sign in, reset your password, or receive verification codes, you can request account recovery (read this support article: How to use account recovery when you can’t reset your Apple ID password - Apple Support ) to regain access to your account. For additional assistance from Apple Security, read: Contact Apple for help with Apple ID account security - Apple Support
If it's another reason then post here with details.
Once in the account this support article explains how to cancel a subscription from Apple on an Apple mobile device, Mac, PC, and online ➔ If you want to cancel a subscription from Apple - Apple Support or this video will only tell you how to do it on an iPhone ➔ https://www.youtube.com/watch/XXx-J-ueCtE
Notes:
• You might need to scroll down to find the Cancel Subscription button.
• If you don’t see Cancel Subscription, then the subscription is already canceled and won't renew. An "Expires" date (instead of "Renews") means the current subscription will just run until then. "Resubscribe" is just in case you change your mind about having it end.
• Deleting an app does not cancel a subscription.
• If you don’t cancel trials more than 24 hours before your renewal date, your subscription might renew.
• If you don't have an Apple device, see the options near the bottom of the support document.
• Many subscriptions are set to renew automatically until you intentionally cancel them.