How do I empty my App Store credit?

You can contact Apple Support and ask that they clear that balance. Here are the steps:

1. Visit Apple Support.
2. Under the See All Products section, click the Choose a product link.
3. Click Apple Account, then select Other Apple Account Topics.
4. Choose Clear Apple Account Balance from the options.
5. Click Continue, and you’ll be shown how to contact Apple via chat.

Hello ~If you are trying to zero your balance before changing regions…you may purchase an item from the App Store for more than your balance and the remainder will be billed to your credit card on file thus zeroing your balance . You may also go to lower left hand corner and click on Support and they can zero it for you or…

Contact Apple for support and service

Also if you are changing regions take a look through this article as well…

Change your Apple ID country or region - Apple Support

Apple Support

~Katana-San~