Alternatively, you can keep the URL and just delete the content and start again. If you don't have a lot of content you don't need to wait for Staff to do this. You can delete the content and retain the URL and start fresh.
bulk delete images & media http://en.support.wordpress.com/media/deleting-files/#bulk-deletion
bulk delete posts > http://en.support.wordpress.com/posts/edit-posts-screen/#bulk-edit
bulk delete pages > http://en.support.wordpress.com/pages/edit-pages-screen/#bulk-edit
Here's how to permanently delete Trash >
http://en.support.wordpress.com/trash/
http://en.support.wordpress.com/trash/#deleting-permanently

Or you can make the blog private and use it as a test blog and backup blog. http://en.support.wordpress.com/settings/privacy-settings/ And, you can hide it from dashboard display as well. http://en.support.wordpress.com/my-blogs/#my-blogs-dashboard-visibility