Ive used the owl decoy to limited success - problem is they learn but it will buy you some time. Like the snake story which was priceless !
Ages ago, a science teacher told us a grumpy old man got tired on having teenagers lean on his car so he hooked up a battery. Everything that touched his car got mild "zap." No idea if it's an urban legend or a lawsuit waiting to happen but it ought to make a good story either way.