I have actually heard this before but inever thought that this could be my own experience. I never knew if the it is the true reason why my sites suddenly got higher rankings but as soon as i removed my sitemaps for my sites,they all ranked well to this day. It was after two weeks and i did nothing else to the site itself. Has anyone have any relevant and similar experience?
Not sure if you can actually stop Google from using your deleted sitmap. Check this: nonadsense.blogspot.com/2007/03/google-sitemap-like-it-or-not-but-you.html
you could try replacing your google sitemap a couple of weeks before deleting it. I don't know whether they cache old deleted sitemaps.
This subject is one I have mixed feelings about....It's tricky. We're currently running some tests on our sites to see. So far, I've seen it go both ways, so I'm not convinced that the xml sitemap is the determining factor. I recently wrote about this in a post entitled, "SEO Expert Warns Against Using Google Sitemaps". Please read and let me know your thoughts.