Campaigns and ads cannot be deleted from the platform, however they Archived. Archiving a campaign or ad will effectively remove it from the dashboard, and prevent it from running live.
Clicking the status drop down in the status column will provide the option to either turn the campaign or ad ON, OFF or Archived. Once you select Archived, the campaign, site or ad will disappear from your dashboard.
You can view your archived campaigns, sites or ads at any time by selecting All from the search toolbar. Keep in mind, the default for this search option is always "All but archived".
This process is the same from the Sites portion of the Detailed Reporting, and from the My Ads tab.