The San Francisco group ForestEthics now has an on-line petition to establish a government "Do not mail" registry.
I heard about this on KCBS radio (also reported on KCBS.com) and assuming KCBS did some fact-checking, this is a legitimate environmental organization.
If you want to sign the petition, go to forestethics.org and click on "Help stop junk mail: Sign the petition." The website also has links to other ways to reduce junk mail.