Done a search to find that people stuck up signs on their postbox saying "Please dont put in any chirashi" (チラシを入れないでください) and the feedback was that it didnt work.
I then read that adding a "24 hour video surveillance"（監視カメラ作動中） reduces the amount a little.
I took a step further and added a "Pictures of people who stick in chirashi will have their face-photo uploaded to the internet."（投函した者の顔写真をネットにアップします）
The result was that the 10-20 chirashi per day was reduced to *zero* chirashi! I'm pretty happy about this as you can imagine ^^;
There is another type of chirashi known as Pink Chirashi which is basically call girl leaflets. I think I got a few in the past but not many - you can see them stuck in phone booths all over Tokyo too.
I saw a picture on some Korea blog (cant remember where) of chirashi being stuck to the bloggers door! Dont know if thats a common thing over there though.