GARBAGE OUT, GARBAGE BACK: Stuffing random artifacts from a messy coffee table or desktop into the postage-paid return envelopes provided daily by traditional junk mail houses is a tit-for-tat resistance tactic that hits the mailers' postal numbers for the price of a stamp while providing a mild moment of absurdist venting for the recipients' misanthropic main boilers. I highly recommend the practice.
There is now an Israeli company attempting to provide an on-line analog, according to the BBC. While it's unclear how ethical or effective it is in actual practice, in theory it's hard not to like the idea. Of course, the on-line version lacks the personal touch that comes from providing Capital One with a fistful of expired cat litter coupons, an unwanted take-out menu and ticket stubs from Batman Returns.