To cancel your portfolio, log in to your account. From the menu at the top right (click on the person), select Account Settings. Select the final menu option on the right (Account >) and click the Cancel My Account link. This will also delete your subscription information.

If you have a premium account and simply wish to cancel your premium subscription while retaining a free account, select "Billing" from the top menu, and you will see an option to cancel your payment.