In the lower right corner, click the dashboard icon (a house on a green background), then click Website Settings on the left and you can cancel your website there.
As another option, you can change the domain and simply remake the website. If you go this route, I would advise making all of your pages set to Administrators for viewing. This will stop anyone from browsing around while you are re-creating it. Just remember to reset the pages' viewing access when you are done.