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Changing/removing payment info : Change or remove your Apple ID payment information in the iTunes Store - Apple Support

If you don't get the 'none' option : Why can’t I select None when I edit my Apple ID payment information? - Apple Support

If you want to remove your credit card from your 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. If you are doing it on your iPhone then tap on your id in Settings > iTunes & App Store and tap 'View Apple ID' on the popup and log into your account- that should also give you a payments link on your account's page.

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

If you just want to prevent apps being bought/downloaded on your phone then you can hide the App Store via Settings > General > Restrictions > Installing Apps 'off'

Or do you mean something else ?

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