Try this if you haven't already heard about it. You get junk mail with a prepaid envelope to return a credit card app or something. Place something in the envelope that doesn't have your name on it and mail it back. Make the people crazy with returned mail. Just be sure there isn't some identifier (barcode) on the back on the envelope.
Don't do it for charities, thats not fair.
Yeah it sucks for the mailman. But it will drive campanies nuts!