More than six months ago I submitted a few sitemaps to GWT for one of my sites. Later I noticed that many of the URLs in the sitemaps were wrongly formed due to a config error in the sitemap generator. I was getting lots of 404 so I removed the old sitemaps and replaced them with new ones. Until today google is still trying to crawl a lot of those wrong URLs from the removed old sitemaps. Since the site is really big I'm getting thousands of 404 errors in GWT and I'm afraid that it may impact the crawl health of the site. Anyone knows how to stop Google from requesting the old URLs?