To the best of my knowledge no local authority in the UK has ever done anything to encourage / force distribution companies to respect 'No Junk Mail' signs. A while ago I did discuss an idea I had for licensing leaflet distributors on the now-defunct Stop Junk Mail Google Group. I did also discuss the idea with one of the local politcal parties. They were interested and thought the idea was feasible but nothing ever came of it (partly due to me not having enough time to keep hassling them).

It does surprise me that local authorities seem fairly uninterested in the junk mail issue. My guess is that many councils (and political parties) aren't sure how to deal with the issue because they're producing unsolicited mail themselves.