While SPAM is relatively cost-free (it's actually a huge strain on mail & other servers), junk mail tends to speed up deforestation. Dunno how good of an argument that is.. I'm mostly just tired sifting through the shit to find normal mail.
Fortunately, a solution is provided by the same people who send us all this snail-spam. Simply put some useless junk (other cc offers?) into the free return envelopes and send them on their way. If everyone starts doing this, maybe these companies will tire a bit from being on the receiving end of their own shit and lay off.
And, as you all may know, numbers hold the power. Show this post to your friends, have them show it to theirs, etc. Let the revolution begin ;)
UPDATE: The idea appears to be extensible. See what someone else came up with: http://www.bash.org/?127039
UPDATE #2: Well, this one will probably work best of all. https://www.optoutprescreen.com let's you opt out of receiving CC offers in the mail. Before using it, though, definitely do read this review: http://weblog.infoworld.com/foster/2005/10/25.html. Many thanks to induke for this one.