From a Gamespot forum:
call them to cancel 18004myxbox
Expect to have a difficult time with customer service… make sure to write down the person’s name that is talking to you on the phone. When you are finished with the phone call, call back and double-check that they have actually cancelled your service. After they tell you they have not cancelled your service ask them again to cancel the service. Write down the person’s name again and reference number of the phone call.
…Check your credit card statement in a month and when you find that M$ is still charging you, call customer service and continue to try and cancel the service.
This could be a good example of a dark pattern, but so many people have this problem I wonder if fraud is a more appropriate word.
Other suggestions are to change your payment method to an invalid credit card number. That’s how bad it is.