I don't think it's rude to correct misinformation. People are spreading this info because, presumably, they care about it. So they should care enough to know that it's false, right? That's why I always reply with a Snopes link and a brief line like, "Actually this isn't true, check out the link."
OK that's the polite reason why I do that. The real reason is, just because Aunt Madge or CoWorker B has too much time on their hands and nothing better to do than forward every bit of nonsense that comes into their box, doesn't mean they should think that everyone is happy to receive the glurge.
I get the occasional snitty reply that says basically "Oooh. Didn't mean to BOTHER you." And I think, "Well, you did. Now you know not to do it again."