How to Delete Payment Information From Your iPhone 4
Your iPhone 4 stores your payment information for iTunes and the App Store. This allows you to purchase and download media and applications from the iTunes Store and the App Store without entering your payment information every time you want to make a purchase. Should you want to remove this payment information from your iPhone, you can access and remove your payment information from the main "Settings" menu.
Tap the "Settings" icon on your iPhone.
Tap the "Store" icon.
Tap the "Apple ID" button.
Click the "Sign Out" button. You have now deleted payment information from your iPhone.