I already had my card details on my sons xbox after buying the full version of toy soilders whilst he had his first month free with the console.
I you look on xbox live currently they offer buy 1 month for £4.99 get 1 month free. My 1 year old son managed to get hold of a controller and buy this.
I then rang xbox up (same day) and asked them to remove my card details so to stop any re occuring bills at full price. they said they can only remove card details if they stop the actual 2 month transaction , which i was fine with but then informed me they could not refund the money either, but the could give me a voucher code for the 2 months, which they have now emailed me, but it is actually a 3 month code.
So I have a 3 month code for £4.99 and my card details removed off the xbox and no re - occuring.
I know this is not going to be to everyones taste and that its a long way around it. but you get the code next day (did this yesterday and got code today via email) and no worrys about re billing.