Right next to my mailbox, I will bolt an aluminium trash can (or rubbish bin if I'm not being a Yank) and put the label "Junk Mail" on it. Somehow I doubt it'll ever fill, nor should I expect many more unauthorised pieces of crap in my mailbox. If this idea were to take off, the advertisers directly would see the senseless spending is fruitless, and hopefully cut back until they stop. Humour aside (that's assuming at least one person found my post remotely comedic) Rakuli's idea is a great one.
What a fantasic idea! Can you imagine if we could convince enough people to actually do this? A
trash can on every mailbox would do many things.
1. Show the pamphleteers (Is the most politically correct thing I've heard so we'll run with that) how useless their current occupation is. It would put their job in a new light, who would want to be delivered a whole pile of trash and told to put one piece in each bin. It would be so much easier to put all of it in one bin. If they see that the pile of ads is just trash that should be binned immediately they would see that it should never be created in the first place.

2. People who walk around residential streets and find themselves with a piece of garbage (bottle, can, wrapper) never have anywhere to put it (unless it's bin night). With a
trash can on every letterbox, they will always have somewhere to place this litter and "Do the right thing"
Of course, this is a little extreme and the best idea is to stop it at its source. You can't punish the deliverers for getting a bit of cash for their efforts -- unless they blatently ignore "No Junk Mail" signs as GolfTango says above.
As soon as I find the time to download and install Open Office, I will add shine_on's letter to councils as a Microsoft Word, Rich Text Format and Plain Text document for download. Then we can start to make this happen.
Petition the Councils! Coles Bay is a perfect example of how the small idea of one person really CAN make a difference.
This is a great example. For those that haven't heard of this, the residents of Coles Bay in Tasmania started a movement and had plastic shopping bags completely banished from their town! A mammoth effort considering the large reliance that modern society has on the humble plastic bag.
Coles Bay -- Australia's First Plastic Bag free townScary facts about plastic bags
To help get this idea underway you can drum up some interest by clicking the "Digg This" button underneath the title at the top of the page. This will show the idea to many potential readers, the more "Diggs" the more readers.