how do i cancel my payment method or change card method to none

when i want to dowload an app thats free it tells me that i still need an active credit card. i only had the card to purchase a couple of songs and now i want to take that card off and not use my card again. before i had purchased the songs, my payment method was selected as 'none' now its on 'mastercard' how do i selesct 'none' again if it doesnt show up as one of my options???????