Instructions
Go to Xbox.com and log in to your Xbox Live account. Click "My Account," then "Membership Level."
Click the link that says "Automatic Renewal: ON." The next screen should ask you to confirm before switching off the automatic renewal function, placing your Xbox Live subscription on hold temporarily.
Call 1-800-4MYXBOX and select the "Customer Support" option. The customer service rep will ask you for account information, including the answer to the security question you created when you set up the account. Confirm that you want to remove your credit card from the account as well, to cancel the Live subscription completely.