Q: Canceling a purchase

All sales are final the moment you click buy.

You can try contacting itunes support and asking for an exception

Contatc support. I fthis is your first accidental purhcase then they might refund it as an exception:

http://www.apple.com/support/itunes/contact/

You said your device did not ask for your password before the purchase. It's probably tirned off:

Resetting iTunes Store warnings

http://support.apple.com/kb/HT1734