I always shred the documents they sent, fill their SASE with the shreddings, then mail it back to them on their dime.
You know, until now, I knew what they were, but I just tossed their crap in the trash can. But to actually abuse us, not just use us, I may take up your return mail practice. Takes a little more time, but not much. :)
Did you ever wonder how they got your name and address in the first place, and knew not to send you mail when you were 16 years old?