Only the owner of the organization account site will be able to delete an organization's user account. If you're not the owner, head over to Assign Roles and Permissions to make sure you are an owner.
Simply go to Organizer Dashboard >> Settings >> More Settings and click "delete" link in the left hand column and confirm (for the one you no longer want.)
(If you are a paying customer, deleting the organization account from settings will not automatically cancel your subscription. If you know how to cancel through your paypal then do so if you need assistance contact us directly.)