Thank you for reaching out to the Community, thats a great question! I did a bit of digging and unfortunately, there is not an option to "lock" the app to restrict people from exiting it. Maybe if you put the List Builder app on its own page on your Kindle Fire, if customers exit out of the app, it will put them on the screen where the List Builder is the only visible app available. This could lessen the chances of people using your other apps on the Kindle!
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