This changes all the time and they deliberately don't make it easy by burying the option, make it effort and generally aren't helpful abot the whole thing.
Phoning up works, but you can also do it from www.xbox.com. To do this togo XBox.com log into your account, select Contact Information, Membership Level and then you will see a bit on the screen that says 'Automatic renewal: On' click the ON (a hidden hyperlink?)
Now just keep clicking next while they try to persuade you to not go through with cancelling your account and you are done. You account won't cancel until your prepaid period expires, but at least they will stop taking money out of your account.