How do you keep strangers from giving your livestock "treats?"

As I'm sure anyone who has a pasture fence sharing a border with a road knows: it's almost impossible to stop people from touching or feeding your animals, and this can sometimes have fatal consequences.

I don't think I know a single person with livestock that doesn't have trespassers because "we heard this was the place with baby goats," "we wanted to see the foals!" or who have had garbage thrown over a fence because "goats eat everything." Even people on private property are not exempt from this kind of thing. Signs and electric fencing don't seem to help. Does anything actually work?