To cancel billing for a specific app (or to cancel a trial) click on the cog at the bottom left-hand side of the screen and at the top on the left click on 'Manage' under apps. Then hover over the app in question and click on the cog like this:
From the drop-down menu, click on "Uninstall".
Important: Note that if you simply disable the app, this will not cancel billing for the app. The app needs to be uninstalled for the cancellation to take place.