You can cancel your Account (deleting all Podcasts and associated data) at any time. Just click the My Account link in the navbar, then click Subscription in the sidebar, and click the red Delete Your Account button.
This will delete all of your Podcasts and their associated data. Please see the section on deleting your Podcast for more information.
If you are still in your trial period, your credit card will not be charged. Only the account holder can can delete the entire account including all podcasts, Collaborators cannot.