using CMS.SiteProvider;

using CMS.CMSHelper;

using CMS.WorkflowEngine;

...

          // Code name of the site

          string siteName = "mysitecodename";

          // Delete records in CMS_Tree and CMS_Document

          DocumentHelper.DeleteSiteTree(siteName, null);

          // Delete other dependencies and the site

          SiteInfoProvider.DeleteSite(siteName);

           lblInfo.Text = string.Format("Site '{0}' has been deleted.", siteName);