No subscription refund possible - how can I get it??

Contact iTunes Billing directly and talk to them about it: iTunes Store - Contact Us - Official Apple Support

Disputing the charge directly with the bank, will generally only end in the charge being blocked, and the card being unverified in the iTunes an App Stores. This blocks access to the stores including downloading free apps and updating existing apps until the card is re-verified which generally required the charge that was blocked to go through successfully and you are back to where you started and is just generally very messy.

Best to go through iTunes Billing directly before involving the bank. They can be persuaded to provide the refund, and maintain the account in good standing as such.

There is a few things you can try. You may want to contact Apple Support and dial 1-800-MY-APPLE, or go to https://www.apple.com/support/ and talk to a representative. If there's nothing you can do, then get in contact with the developer again and be persistent. If that doesn't work, then try dispute it at your bank.