Outdoor Animal Control - dog poop problem

Out by our maibox next to the sidewalk along our neighborhood street on the corner of our yard we've been having a problem lately with somebody either letting their dog or perhaps a stray dog poop there and leaving it. Never seen when it happens but just see the results and am getting tired of having to clean it up. What's an effective and humane way of keeping this from happening, and I'm not going to build a fence. Some substance perhaps that I could put on the ground around there. I've heard of using cayenne pepper, and have tried it but it seems like whenever it rains it must get washed away or something, because the problem continues.

I see you have also posted this in the Pets forum so I won't repeat what is here.
http://forum.doityourself.com/pets-farm-animals/385916-dog-poop-problem.html

Anything you spray or put on the ground that is organic will need to be replenished after rain. Here's some other suggestions.
Critter Trouble (http://www.ghorganics.com/page6.html#CATS%20and%20DOGS)

A scarecrow sprinkler would also work and you could probably safely set it at a distance that would be close to the house. Only problem would be when your letter carrier comes to deliver the mail or you go to retrive it .
Contech Scarecrow - Motion Activated Sprinkler (http://www.biconet.com/critter/sprinkler.html)

Newt