It might not be the most efficient means, but it certainly makes me feel good. When ever I get junk mail with a prepaid envelope I simply stuff all of their crap into it and send it back to them. Sometimes when I get multiple junk mail in one day I mix 'n match.
Of course there are websites out there where you can opt out of that stuff (more details at my blog post http://www.russellheimlich.com/blog/how-i-deal-with-junk-mail/
) but this method isn't that bad for the small amount of junk mail I do get.