All purchases are considered final, but you can try contacting iTunes support 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 don't want to be able to make in-app purchases then you can turn them off on your iPhone via Settings > General > Restrictions > In-App Purchases 'off'. You should also be able to set 'Require Password' on the Restrictions screen to 'Immediately'
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