I had this from Apple... didn't work for me but may for you:
Here are instructions to delete your credit card information from your iTunes Store account:
1) Make sure you're using the latest version of iTunes. It can be downloaded free of charge from the Apple website at:
http://www.apple.com/itunes/download
Note: Installing the latest version of iTunes will not affect your library or any items in your account that you haven't downloaded.
2) To launch iTunes and be taken to your Apple Account Information page, click this link:
https://phobos.apple.com/accountSummary
(If you are not already signed in, you will need to enter your iTunes Store account name and password and then click the Account Info button to sign in.)
3) The Apple Account Information page will appear. On this page, click the Edit Payment Information button, which is the second button from the top.
4) On the Edit Payment Information page, select "None" from the list of credit card types. This will delete your billing information.
5) Be sure to click the Done button at the bottom of the page to save your changes.
Now that has been removed, I need to you go to appleid.apple.com to make sure all your security questions are set up properly.
1. Select Manage Your Account.
2. Enter your iTunes username and password
3. Select "Password and Security" from the list on the left.
4. Change security question and answer and make sure your security question are correct
5. Click "Save Changes".
Once that is done, I need for you to try making another purchase (you will be asked to add your credit card) - at which time you can add your card and complete the purchase.
I hope this clarifies your concerns.