When you log into your Bravenet Account Manager, you should see a link that says "change information/password". If you follow that link and then scroll to the very bottom of the page, you should find a button that says "Delete Account". Just press the button and follow the instructions.
Note that this will delete everything in your account. Your web hosting and all of your services. If you only want to delete your web hosting, use the "manage hosting" link and then follow the "delete website" link. If it's a service you want to delete, navigate into the service's managment link and use the "delete service" link.
