Help

To remove a card from an account : Change or remove your Apple ID payment information in the iTunes Store - Apple Support 

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

You are just logging into the account, not trying to do it when downloading something ? If you don't think any of the scenarios on the link that I gave apply you could try asking iTunes Support : http://www.apple.com/support/itunes/ww