I got an email one time about how to deal with junk mail. It talked about how, because of the internet, the U.S. Postal Service is losing money. I don't know for sure if that's true, but judging how many times postage has gone up in the last few years, I would guess that it is. Anyway, it said to help the post office out and annoy the people who annoy you with junk mail, just send the postage paid envelope back to them.
I know someone who does this and puts random things in the envelope like dryer lint. While that's kinda funny imagining someone opening an envelope with dryer lint in it... I like to be courteous and issue a reply. I always take the credit card application enclosed, tear it off and write with a large Sharpie, "NO, THANK YOU!" on the application. They always ask me to respond by a certain date, so I try to honor that. :P
It always makes me laugh anyway... I feel good about giving the post office some revenue and sending so many annoying junk mail things back to the sender with my reply! I did this yesterday... and I sent out 10 NO THANK YOU replies. I felt good about that.
I don't know if this helps the post office or not... but it sure entertains me!