You can just set the status back to "Draft" and it will not appear any longer in our sitemap files or onsite. The steps to do this are:
1. Select the site you wish to set to "draft" from the Sites screen in the Account section
2. In the dashboard, click on the Setup icon
3. On the Setup > General screen, you will see an option for Site Status. Simply toggle that setting back to Draft.
We also suggest that if you have setup a CNAME or A Record with your domain host that you delete this. This will ensure that your URL does not try to redirect to LaunchRock any longer.