The Melt Shop app is powered by LevelUp, one of the most secure payment systems out there, so preventing unauthorized use of your account is simple. All you need to do is de-authorize your device, and the 2D barcode on your app will no longer work.
To de-authenticate your device:
Log into your account at www.thelevelup.com
Click on Control Panel at the top of the page
This will reset your barcode and log out your account on all devices.
To access your new barcode, you'll have to log into the app again.