How do I get rid of xbox 360 credit card information?

I used to use my parents, but now that I have my own credit card I want to delete theirs. I already have my credit card information on my xbox 360, but my parents credit card is still the default card so I keep on accidently using theirs instead of mine. So I want to get rid of it completely...|||http://www.gamespot.com/pages/forums/show_msgs.php?topic_id=25452161