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.