1. Open the envelope and look for a pre-franked reply envelope. If there is one, stuff the junk into that and post it. Extra revenue for the postal service.
2. If it is a really persistent and annoying sender, mutilate my address and post it. This should result in it being routed back to the sender but does impose extra work on the postal service.
Oh yes, I've done #1 before. My favorite is to stuff a credit card A offer return envelope with the crap from a credit card B offer, and vice versa. Sometimes if they really piss me off, I'll throw in a few heavy things (rocks, etc) so they pay extra. A small ziplock bag full of sand fits very nicely, and weighs (relatively) a lot. I've yet to try taping a return envelope to a large box, but it seems like the postal service would accept it and charge them accordingly, right?
I'm curious if there's a list of things like this that the USPS explicitly does not allow..