All purchases are considered final, but you can try contacting iTunes support and explain what happened and see if they will refund or credit you : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Purchases, Billing & Redemption
If you haven't already done so, then to try and stop it happening again you can turn off in-app purchases via Settings > General > Restrictions > In-App Purchases 'off', and you can set 'Require Password' to 'Immediately' on the Restrictions screen (the default is a 15 minute period during which it doesn't need re-entering).