Send them back with your own editorials, left over campaign bumperstickers, election pamplets, NRA literature, etc...
I get environmental and political junk mail and find magazines with postage free business reply envelopes for left wing looney groups. They all get a standardized letter I have photocopied for just that purpose... OR, just fold up all their crap and send it back to them...
"OR, just fold up all their crap and send it back to them..."
Ah yes, the old folding all their crap up and mailing it back to them trick. I do this with the Sierra Club. I Somehow ended up on their mailing list for a few years and made a habit of mailing thir crap back to them in their own envelope. The amazing thing to me is that for a supposed "environmental" group, they mail out 4-color stickers, calenders, slick letters - all the bells and whistles. I guess the environment matters to them as long as it doesn't get in the way of their fund-raising.
I see these kinds of tactics as a sort of ultimate FReep where the object of derision funds the protest. Imagine millions of FReepers doing this.