This one's easy. You can't cancel via email because then anyone could forge an email and ask us to shut down an account. Naturally, that would be bad.
So, assuming you're the account holder, you'll just need to login and go to the "Your Account" link at the top of any page then to the "Plan & Billing" tab. Then you'll see an option to "Close Account".
From the "Plan & Billing" tab go to "Close Account" in the sidebar, read through the page and if everything sounds good click the link at the bottom to continue to the final cancelation page.
The last page confirms that you're the account holder and really want to cancel your account and also provides a chance to give us some feedback which we truly appreciate.