From their site:
Sign in to your Admin Console. Make sure you are signed in as the owner, as that email address is the only one with upgrade/downgrade privileges.
Click on Settings in the Admin Header.
Choose General from the drop down menu.
Scroll down to Billing & Invoices. There’s a link that says: Delete my account. If you’d like to delete your account (which will get rid of all your data and forums), click this.