Disable buying from itunes

From this thread:

https://discussions.apple.com/thread/4062900?start=45&tstart=0

"Hi! There, actually there's a way to remove your credit card information to "None." But you've to do it with a computer. Go to computer iTunes and click on iTunes Store, then click on your account. Key in your Apple ID passwords then edit the billing information and you shall see the "None" option. This is a tricky part where you need to use computer's iTunes instead of your iDevices App Store. This worked for me, hope this help you too and have a nice day."

You can't have disabled the iTunes Store and still get a free app, so I'm not quite sure what you mean. In regards to the charge, though, you probably inadvertently made an in-app purchase. Many apps, particularly games, are free for the base game but charge for additional features such as levels, "coins" and other such add-ons. First, to prevent this from happening again, turn off "In App purchases" in the Restrictions settings on your iPad. For more information, see:

http://support.apple.com/kb/HT4213

As to a refund, that's not automatic since the terms of sale for the iTunes Store state that all sales are final. You can contact the iTunes Store, explain the reason for your request, and ask, though:

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

They're usually pretty lenient in the case of inadvertent purchases, but there are no guarantees. Definitely contact them if you are certain you could not have made any in-app purchases.

Good luck.