Have you looked at the archive functionality? It allows you to archive a site (aka ‘site is not live’) and unarchive it again when the site is ready to go online. You may do so in ‘/wp-admin/network/sites.php’. The option is shown when you hover over the row with your site in it.
Another option is to make the site only available to logged in users. You may use Justin Tadlock’s Members plugin for instance to achieve this.