All purchases are considered final, but you can try contacting iTunes support and explainwhat happened 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
To try and stop it happening again you can turn off in-app purchases on your iPad via Settings > General > Restrictions > In-App Purchases 'off', and (depending upon what version of iOS that you have on it) you might also be ableto set 'Require Password' to 'Immediately' via the Restrictions screen.
Did it fail to function? Did it not download properly? Why do you want the refund?
All sales are final (as far as the word goes) and it is part of the Terms and Conditions that you understand this. Having said that, my experience with App Store has always been positive and if you have a genuine reason to ask for a refund, it will be considered, I am sure.
But 'don't like it, can't use it,' won't wash, I'm afraid.