I used to get those scam letters a long time ago- back when they were stupid enough to include a pre-paid return envelope. I always used it to return the letter. After the first few letters, I started writing nasty notes to remove me. They kept coming, so I started writing longer notes pointing out the economic loss of continuing to send me letters (postage both ways). They didn't stop, so I started including extra scrap paper, just to make sure the weight exceeded the normal amount and they would have to pay extra. Pretty much finally stopped. :)
However, I still get the occasional message from a client who gets a letter form them.