How can I empty my Apple ID balance to zero?

First, cancel your subscriptions and wait until the end of the subscription period.

Second, make a request to zero out any remaining balance on your account by contacting Customer Support by phone or chat:

• Chat now or call 1-800-APLCARE.
• International Support.

—Then—

Click ▶︎ Change Region. 🌐

Buy something more expensive than your balance and use a different payment method from the current region for the extra amount; if you can't, click here and ask the iTunes Store staff to zero it.

(252982)

Follow the directions in Niel's reply. It provides the direct link to the people at Apple handling this sort of thing. If you use links to Apple's general support you will find they just take you there anyway after spending 5 minutes trying to find what you need on the web site.