I figure since these companies have no compunctions about wasting my time and mailbox with their crap, I should return the favor. Had I small envelope-sized plates of steel to put in the envelopes instead, I'd use those, but I don't - doh!
amigaboy
The envelopes from these people are almost always reply paid (obviously meaning that the original sender pays for the return postage). Get a large box and fill it with crud and then tape the reply paid envelope to the top of it. Not only will they be forced to pay for the excess postage but they also get to enjoy delving into the depths to discover what you've sent them. Newspapers and banana peels work a treat.