My husband and I have been doing the envelope stuffing thing for 25 years. Sure has cut down the junk mail. Sometimes we sent back 5 letters in one day. When people were really persistent, I wrote in there that I would send them a box of rocks if they continued sending their stuff. That worked! One person actually sent a box of rocks to an address in Alaska. The addressee had to pay for it because it had the postage paid return envelope taped to the box.
About 10 years ago, this idea was on the Andy Rooney show. It had already been working for us for many years. The hardest ones to get rid of were the book clubs.