Can I cancel purchase on iTunes?

Purchases are considered final but you could try the 'report a problem' page to contact iTunes Support and see if they will refund you as a one-off : http://reportaproblem.apple.com

Or you can try contacting iTunes Support via : http://www.apple.com/support/itunes/ww

Get help with an item purchased from the App Store, iTunes Store, or iBooks Store - https://support.apple.com/HT204084

Report a Problem to request a refund - http://reportaproblem.apple.com - "Sign In for help with iTunes purchases made in the last 90 days"

If a reporting a problem request is unsuccessful you can also try:

Other ways to contact iTunes Support:

https://www.apple.com/emea/support/itunes/contact.html

https://support.apple.com/country-selector/itunes

Apple's terms of purchase for most countries are effectively, "All Transactions are final" (check terms at: http://www.apple.com/legal/internet-services/itunes/). Apple does refund on a case by case basis, but realize they can also say no.