When I don’t outright refuse junk mail, sometimes I’ll take some of the junk mail I’ve received, remove personally identifying information from it, and stuff it into the next available Business Reply Mail (postage will be paid by addressee) junk mail return envelope I find, and drop it in the mail. I hope to exceed the one ounce limit so that they need to pay extra postage.
I’m just very sick of junk mail. I don’t allow random offers from companies to show up in my email box, so why should I need to deal with them in my postal mail box? Unfortunately, setting up filters on your physical mail box is much more difficult than setting them up on an email box. (It is possible, however, and I plan to do so in the near future if I continue to get more junk as companies discover our address.)