How do I completely delete an app that pay for? And make sure I am no longer paying on it?

Go to App Store on iPhone

tap on your apple id

tap on view apple id

tap on subscriptions

You should be able to tap on the subscription you want to cancel.

As for permanently deleting the app off your account, I believe you have to go to iTunes on a computer and remove them from your account otherwise they will just show up in your iCloud.

Hope this helps.

Apps are a one time purchase, as such there is no ongoing payment for them.

You may have activated a recurring subscription within the app for some service it offers. In which case you would need to cancel the recurring subscription by following the steps in this support document:

View, change, or cancel your subscriptions - Apple Support

Once that is done, you can confidently delete the App in question:

http://help.apple.com/iphone/10/#/iph248b543ca

See your purchase history in the iTunes Store on a Mac or PC - Apple Support

View, change, or cancel your subscriptions - Apple Support