I found this along time ago, and it is SO AWESOME! http://www.vertical-visions.com/_temp/postagepaid/index2.html
thats awesome. I'll be trying that this week, as I have a p.o. box in mound but still get spam at my address here.
Wow. Never thought of going that far. Back in the days of the anthrax scare I sent back plenty of envelopes filled with sugar/salt/flour/whatever.
I think that works. An older coworker of mine has told me that my company (insurance) has received boxes of rocks, junk and even hair from prospect customers. lol!
Used motor oil is a hazardous material, so that could actualy land you in deep ****. Likewise MethodlesS' idea is a Biohazard. I had a friend who used to keep a sheet of steel and a tinsnips near his couch. While he was watching TV he'd cut chunks of steel and mail those back in the postage paid envelopes.
My old roommate used to mail back the other junk mail from other companies all of the time. Then we'd get drunk. w00t!