How can I cancel a purchase that is still pending?

Purchases are considered final as you know, but you could try contacting iTunes Support and see if they will cancel it as a one-off : http://www.apple.com/support/itunes/ww (click on your country's flag, the country where you live, and which is therefore the country on the account ; click on Contact Support towards the top right ; click on Get Help under 'Talk To Us’)