You can delete all content and retain the URL.
If you don't have a lot of content and don't wish to wait for Staff to delete it for you then you can quickly remove your contents by deleting them in bulk.
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 >
Alternatively, if you do have a lot of content, you can tag this thread with modlook in the sidebar and Staff can empty the whole blog for you. That will remove all data including posts, pages, tags, categories, comments, and uploaded files and once deleted, the data cannot be recovered. If you are sure you want Staff to proceed, please reply to confirm the URL of the blog you want them to empty.
IMPORTANT NOTE: Google and Bing only clear their caches of deleted indexed content that produces a 404 (page not found) every 3 - 6 months.