I assume you have viewed and cancelled all subscriptions managed by Apple:
If you want to cancel a subscription from Apple - Apple Support
If you owe money from a previous purchase/subscription, then that will be charged when the money becomes available. This will continue until that obligation has been met. Review this Support Article to view you purchase history and also use the App Store to view history for older purchases.
View your purchase history for the App Store and other Apple media services - Apple Support
Depending on the exact description of the charge, it may also be a subscription that is not managed by Apple and was made with the Developer directly. Those can only be cancelled by going to the website for that developer where the subscription was started.