Any excrement works nicely. I used to have an electrical transformer box mounted in my front yard, and the local cretins repeadedly jimmied it open and shorted out our power (whatever pleasure they got out of this was more than offset by the danger they were exposing themselves to). I stopped it by depositing some of our dog's droppings around it instead of in the yard waste can for a few weeks.
True, but there's something particularly base and vile about human crap. Just knowing that someone went through the effort of using their own excrement as a weapon/deterrent goes a long way toward putting some real fear in people's minds over your mental stability.