You delete an Item by sending an HTTP DELETE request to the items resource.
Items are not 'physically' deleted - instead, the (read-only) status field is set to STOPPED. The Item will no longer appear in the Campaign Items list, and will not be served as an ad.
Once an Item is deleted, it will stop appearing in the list of Campaign Items. Trying to fetch/update the Item will result in a 404 Not Found error.