How do I cancel my iTunes billing account

If you mean remove your card details from your account (you can't delete an iTunes account), then on your computer's iTunes you should be able to edit your payment info by going into the Store > View Account menu option and logging into your account, and on your account's details page there should be a payment link.

Changing payment info : http://support.apple.com/kb/HT1918

If you don't get the 'none' option on your account then do you owe iTunes anything or have any pre-orders/subscriptions due : http://support.apple.com/kb/TS5366 ?