I just had an email from a client and though the information might be useful you, too.
Question: How do I cancel a MobileMe (formerly .Mac) account so it does not auto-renew?
Answer: I found this document on Apple's site explaining the online process.
Canceling your MobileMe account
Since this is a sales-related issue, you might be able to call the Apple Store at 1-800-MY-APPLE, but I am not sure.