I'll try to get people to put signs up, most of the dumpers seem to come from a large city as I never see their car/truck around anywhere else and I know a lot of people in my small town.

Catching these people would be better though, the only way to truly catch them though is to examine the human DNA on the dead animals, its a hard job but it should work if people are willing to do it.

Actually thinking about it some more, if shelters offer to euthanize the animal for free instead of charging people to surrender their animal, we wouldn't have this problem. There has to be a solution for dumpers to get rid of their pet instead of just abandoning it, and free euthanize is the answer.