So you want to jump on the "I'm not renewing .mac" bandwagon, eh? It only took me three attempts over the course of two days to figure out that following the "click here to update your Renewal Settings first" link in the auto-renewal notification email followed by clicking on the prominently displayed "Choose your renewal settings" link only allowed me to renew. But wouldn't you have clicked on this link, too?
At this point, whatever my reasons for not renewing were have been replaced by "It shouldn't have been this tricky to disable auto-renewing."
Admittedly, the checkbox is only one page away from that "Account Settings" page (under "Credit Card Info," obviously), and it turns out they cover it in the .Mac FAQ, but that doesn't make me any happier about it:
Q: How do I turn off auto-renewal and cancel my .Mac membership?
To cancel your .Mac membership for the following year, go to Account and click the "Credit Card Info" button. Uncheck the "Auto-Renew" checkbox and click "Submit". The main Account page will then show that your account is set to expire on your anniversary date. Your credit card will not be charged for the following year, and you will receive a notice on your anniversary date that your account has expired.