If you buy XBL with a card, your subscription is not set for renewal, and as a result, once it ends, it ends. No further action is needed on your part. If you pay with a credit card, it's likely to be set with automatic renewal on, and you have to call to cancel that.
And yea, I really don't like the ads and shit all over NXE. They've got an entire tab devoted to ads once you log onto XBL. Annoying as hell.