It would seem you have heard nothing from the neighbours at all, so is it unreasonable to assume that they are not aggressive or unhelpful, just non communicative? This could be for many reasons (Maybe someone in the house is ill and needs a lot of attention, or some similar scenario, and the fence is the last thing on their minds.) Contacting 'The Police' seems a heavy handed route, and as they evidently don't even respond to CCTV images/number plates of villains actually handed to them (lack of manpower - no criticism here) I can't see them rushing round to sort out some dog poo. I would just put up a sturdy barrier, either a fence panel or anything suitable, and sprinkle some hot spice as suggested around the area. The worst thing is to fall out with neighbours but you do seem to have an absolute dread of people that you haven't seen or heard from as yet - bad past experience is very hard to get out of your head isn't it?