I've always found the council to be very responsive when I've let them know about things like this (they even let you use an iPhone app to report litter etc).

Personally, I don't think the stencils do any good. They quickly become indistinct, and I don't think that a dog owner is ever going to look at one and think "Oh, this must be one of those footpaths I'm not meant to leave mess on" and change their approach.

And we're left with a mess of yellow paint which the street cleaning people can do absolutely nothing about.

To the poster who asked if it was a big concern for the area, I really don't think it is. Certainly Lewisham makes more of an effort on these sorts of things than a number of other London boroughs (based somewhat unscientifically on my own observations while walking around).