To stop an app subscription simply remove the app from your site. The subscription will be automatically cancelled at the end of your next monthly billing period.
If you wish to stop a subscription for a legacy app installed before June of 2017, please go to My Settings at the top right of your Cloudflare dashboard
From here one can select "My Products" and select the App subscription you wish to cancel.
Note: Turning a legacy application off does not cancel the subscription. Please be sure to cancel the subscription to stop billing for an application.