Brief Description of Tip:"Return Postage Guaranteed"
Tip on fighting back:
Ah, these three words on an envelope will mean lots of opportunities!
What "Return Postage Guaranteed" means it that the receiver will have to pay for the postage on that envelope (and whatever is inside it or attached to it) when it gets to them.
So, first off, if you are not as adventurous at first, start by tearing up the ad they sent you and putting it in the envelope and mailing it back to them. Or just mail them the empty envelope.
Or make fun of their product and/or job on the ad. Example, I got an ad for Bosie (overpriced stereo equipment) in the mail with an envelope that was marked "Return Postage Guaranteed." The ad had a picture of their product that had "What if..." printed next to it. So I took a label and stick it right after the "What if..." and wrote on it, "you stopped mailing people this shit and got a real job!" Folded the ad up (so it would fit in the envelope) and mailed it back to them.
As you get more adventurous, (and have more garbage to get rid of) start taping the envelop to the items you want to get rid of. I recommend heavy items such as rocks and sticks, but use you imagination.
Take a box, fill it with old items that you were going to throw out, then tape the envelop to the box. Bring it to the post office. They will happily send it to the company for you (and charge them for the shipping)
Couple of things to note: do NOT send items that are contraband or illegal to mail. These items include, but are not limited to, perishable food, CDS (drugs), explosives, contagious waste, or live animals.
Note: make sure the items can't be traced back to you. There is no legal recourse the company can take against you, but why give them any hint on who you are?