there is one way to remove CC info from your account, though it is drastic, report the card lost/stolen to your bank,
it will break the CC routing, and during your next purchase (of anything) it will fail the transaction, and leave your account in a negative ballance. then clear CC info from a itunes on OSX/other non moble OS (laptop/desktop)
then it should let you select "none." note: you will have to correct the negative balance with an itunes giftcard to continue to recieve OS/app updates, or even to use the appstore at all.
i had to do this as it was just way to easy to nickle&dime your self to death with stupid apps that never seem to support full functionality, alas spaceball2 the quest for more money is all about apple...
hope this helps someone.