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 "Settings" link at the top of any page then to the "Plan & Billing" tab. Then you'll see an option to "Close Account" towards the bottom of the 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.