You can unsubscribe to National Geographic by login to iStore. I had the same problem. You would usually get an email from apple when they deduct money for the subscription at the month end. There should be a link to Go to iTunes. click on that. When you go to that page, it asks you to manage your subscriptions. Click ok and then swicth off the auto renewal of the subscription.
Dear XXXX,
Your subscription to National Geographic Magazine-International will automatically renew on 13/03/12
To cancel auto-renewal or manage your subscriptions, click below and sign in to your account.
Go to iTunes.
Regards,
iTunes Store Team
Hope that helps...