Purchases are considered final but you could try the 'report a problem' page to contact iTunes Support and see if they will refund or credit you : http://reportaproblem.apple.com
Or you can try contacting iTunes Support via : https://support.apple.com/contact or https://www.apple.com/emea/support/itunes/contact.html
If it's a subscription that you've subscribed to you may also want to stop its auto-renewal.