Just as this app has no sign-up or membership registration, there is no account to cancel.
We never send the data you enter to our servers, so there is no problem in simply leaving the app on your device.
If you want to remove the app, just uninstall it from your home screen or app list, and that's it. When you uninstall the app, the notes stored in it are deleted from the device as well.
However, if you are on the Premium Service's "Monthly" or "Annual" plan, uninstalling the app does not cancel it. Please cancel from your subscriptions in Google Play (see "How do I cancel (stop automatic renewal)?" for the steps).
We're sad to see you go, but we're glad you used the app. We hope you'll remember us if you ever need it again. Thank you very much for using our app.