There is no "revert to free version" or even a "cancel account" anywhere on the website so you'll have to actually e-mail them to cancel. Absolutely no other company does this! Not only that, but you can expect to have to submit a lot of documents (government ID, bank statements, etc.) and fill out forms in duplicate (not kidding) in order to get anything done, ESPECIALLY payment info changes and cancelation requests.
Don't even bother trying to remain in good standing with this abysmal company and just request your bank to chargeback any future charges under the name "1&1 MAIL MEDIA". Trust me; you won't miss them.