Mail 'em back
Ever buy or subscribe to a magazine and when you get a copy have to plow through half a dozen or more, postage-paid subscriber reply postcards? Pisses me off to pay for a subscription and then constantly get hounded to buy a subscription with all those little cards falling out from between the pages.
Solution: drop them in a mailbox! If enough people sent millions of those cards back, publishers would be forced to reconsider the economics of placing so many of them into magazines for which you have already paid.