As I’ve mentioned many times before, if you have a product which is no longer available, simply deleting its page from your website and walking away is a bad thing to do. Either redirect the URL to a replacement product, or – if the page is still pulling in decent search engine traffic – consider just adding a message explaining why it’s no longer available and linking to the replacement. What you don’t want is for search engines to continue sending a stream of people to a ‘page not found’ message until they finally recognise that the page is not coming back.
However, there are occasions when you might want to take a page out of the search engine indexes. If so, you can ask them directly; with Google, use the Removals Tool and with Bing, see how to block URLs. Note that this process just ‘hides’ the result on the assumption that you’re going to take more decisive action. You’ll need to take that action, which will normally be ‘301’ redirecting the URL to a replacement. It’s true that as the page will continue to be crawled in the meantime, if you do just delete it the search engines will know not to restore the entry – but other sites won’t have that information, and ‘link equity‘ will be lost.