@NZ Jeremy: The Chengdu Zoo has such a (quite graphic) sign: do not feed animals, do not harass them, don't sleep on the benches, don't pee into the hedges, don't run around without a shirt on...doesn't help.
A German zoo had a smart pun written on its glass tanks: "Nur Bekloppte klopfen", with means "Only schmucks knock". Seemed to have helped at least a little bit...
Reminds me of a sign at a certain public swimming pool which reads:
WELCOME TO OUR OOL
As you can see, there's no P in our pool. Please keep it that way.