This problem was solved ages ago in Holland. Many people have a sticker on their letterbox stating they don't want any unsolicited/unaddressed mail. Actually, there's two versions, depending on wether or not you want free 'newspapers'. There's a no/no and a no/yes sticker.
A bit like this UK initiative:
[img]Download Failed (1)[/img]
[img]Download Failed (1)[/img]
All that's needed is for people to take this up and actually start putting these stickers on their letter box so word gets out.
http://www.stopjunkmail.org.uk/
Either way: it might be worth a punt at £1 for a sticker and 50 P&P for any number of stickers?