You can actually put yourself of a "delete" list for junk mail senders through the Direct Marketing Association. It's worked pretty well for me since I signed up a couple of years ago. I don't get unsolicited credit card offers anymore, and, while I still get some random junk mail, I definitely don't get as much as I used to. Anyway, I'd recommend it.
You can go to the Mail Preference Service website and follow their directions to get yourself on the "delete" list.
For other junk mail that keeps on coming after your register with this service, you have to call the individual companies and ask them to remove you from their mailing list. You can also cut down on junk mail by carefully reading the privacy policies that various financial institutions send you. There are usually instructions in those policies on how to ask the company not to sell your name and address to other companies. It can be a bit of a pain in the butt, but so is getting junk mail.
I guess there's also a pay service, which Matt Damon was talking about on Oprah today, called Green Dimes. I'm not sure if they're better than just doing it yourself, but it's good that there are several options for stopping the flow of junk in your mailbox.
I wonder how much of an impact today's green Oprah will have on her viewers' daily lives. Of course, the main way to be truly green is to reduce how much you're consuming, and Oprah pretty much embodies the opposite of that. She appears to mean well, but she's the queen on conspicuous consumption. Overall, being green definitely appears to be going mainstream. I wonder if it's just a trend, or if people will really adopt their new green habits permanently.