Blogadilla has a hilarious post about how to get back at junk mailer. Apparently, one can take one of those "No Postage Necessary" envelopes, put it on a box filled with bricks, and mail it. The junk mailer will then have to pay something like $0.20 an ounce.
The only catch to this is that since 9/11/01, all packages must have a return address before the post office will mail them. So, the junk mailer will know where the package came from, and so may either take you off of the mailing list, or may retaliate with more junk mail, albeit without "No Postage Necessary" envelopes.