When someone sends a silly chain mail that's one social gaffe, but a small one.
When someone sends
any mass mailing with your address visible to the other recipients it is a larger social gaffe.
In any case, I like to send a polite heads up. Hell, most people went through a phase like that on the internet when they were emailing all the digital contacts they had with all this newfangled digital stuff they found while often not really realizing that if they, in their technical ineptitude have stumbled across it that means everyone else has already heard it or is about to hear it from some other new internet user.
When it's a false email claim I usually like to send a simple link to a site that does a good job explaining it.
When it's a chain email I will privately ask them to not do that with my address again and if they fail to do so I will reply all (an internet gaffe unto itself unless you are fighting the good fight against the much more stupid email-the-contact-book gaffe) with a short explanation of how the spam is harming the productivity of others.
When possible, I like to simply provide links to nice polite sites explaining basic email etiquette. Most people doing this have no idea that they are being rude, and that it's not just a personal issue I have with them but akin to commonly known rudeness like calling someone late at night.
There are quite a few sites that do a good job at organizing basic etiquette in email use, here are some very basics:
http://office.microsoft.com/en-us/outlook/HA012054101033.aspx