You can empty your site of all content by either manually deleting all posts, pages, categories, tags and media, or you can have staff do it for you. Instructions here: http://en.support.wordpress.com/empty-site/ Note that this cannot be reversed. Once the content is gone, it’s gone.
Alternatively, if you don’t want to start over but merely don’t want the site viewable online anymore, you can simply set it to private. You do that under Settings > Reading on the dashboard.