Thank you for your feedback.
Canceling an active subscription to Decibel is very simple. All you need to do is go to the app's Settings (on the bottom right corner of the app's main screen) and scroll down to Manage subscriptions. This will take you to a list of active subscriptions you have on your device. From there, simply select the subscription you want to cancel.
If you have any trouble finding and canceling a subscription, please use Apple's official guide here: https://support.apple.com/en-us/HT202039
Wish you all the best!