there is no option"none" on the payment method

i cant seem to find the "none" option on the payment method page, only visa. mastercard sign and amex. i want to erase all previous payment method and start all over again. This is because i have been trying for a long time to pay a bill on the itunes store and no matter the the type of card used, i receive a message that the security code is invalid.please help becoz i am unable to update any app on my ipad because of this pending purchase