I don't think you actually want to do that. Blocking in robots.txt will prevent Google from crawling the URL - not from indexing it. You actually want them to c

I don't think you actually want to do that. Blocking in robots.txt will prevent Google from crawling the URL - not from indexing it. You actually want them to crawl the URL and then respond with a 404 or 410. If there are inbound links pointing to that URL, you should disavow them in GWT.