how do i get rid of leftover credit in the appstore

If you don't want to spend it (e.g. by redeeming another gift card or adding a valid payment method) you could try contacting iTunes Support and ask them to remove it : https://www.apple.com/emea/support/itunes/contact.html

If you can’t change your iTunes Store country or region - Apple Support - https://support.apple.com/HT204447 - "If your store credit is less than the cost of a single item on the iTunes Store, App Store, or iBooks Store, you can add a credit card to your account [https://support.apple.com/kb/HT201266]. When you buy an item, you will use the remaining store credit and charge the remainder of the purchase to your credit card."

Or:

"If you don't have a credit card, contact iTunes Store support." Apple will remove the balance and keep it.

iTunes Store support. https://expresslane.apple.com/GetproductgroupList.do?&PRKEYS=131629

or

iTunes Customer Service Contact - https://www.apple.com/emea/support/itunes/contact.html

You should only be doing this change as part of moving to a new country. Some people just want to temporarily use the store in another country for access to certain materials but that doesn't work.