If you go to
https://billing.microsoft.com and log in using the Windows Live account linked to the Xbox Live account, you can cancel it on there.
If you have more than 1 paid Microsoft service, you might need to change the dropdown box until you find the one for Xbox Live, but after that, click Xbox Live Gold Monthly Membership down the bottom and then "Cancel my Service".

nope, have to phone up.

had the rolling month option with the ex's card, kept it going for about 8 months before the payment was declined for 3 months - so was owing £15. phoned up support and told them i couldn't redeem my 12 month card, the guy removed the card, stayed on the phone while i put in the 12 month card and said i'm sorted. didn't even take off the 3 months that I owed - might be worth a shot (get your bank to decline the payment):lol:

my ex has been paying mine for months as i couldnt figure how to turn it off lol.

log onto xbox.com and turn it off

Any way of stopping is, other than calling their support line and being on hold for 6 hours?