Cancelling the subscription for the Barcode Scan App is easy. Go to the subscription page in the app by pressing the red button:

Then scroll to the bottom of the page. There you will find an overview of the active subscriptions. Press the Cancel button to stop the subscription.
Revoke permissions from Exact Online
From Exact Online you can revoke the permissions to access your administration. To do this, click on your name in the top right corner and select “My Exact Online”:

Then choose Security Centre in the left menu:

Find the Barcode Scan App there and click on revoke:
This allows you to revoke the permissions you have granted to the Barcode Scan App. The app will then no longer be able to access your administration in Exact Online.