Only problem with that is the perp may not be allergic to poison ivy and it’s doubtful they’ll make the mental connection so it will have no preventive effect. Plus, no one is on their first exposure to it, so it’s unlikely to get results if the perp is a kid whose life consists of school and video games in the kitchen. Others can roll in it on every camping trip and not have a reaction until, say, the 51st time.
Now ghost pepper juice in vegetable oil?
The electric fence is a great idea- instant correction- and it won’t hurt them.
But even a thick coat of petroleum jelly or tanglefoot might be fun- they’ll get nice and sticky and stay that way until they can wash their hands.
“it’s doubtful they’ll make the mental connection “
That’s okay! They’ll be suffering anyway. Caught my neighbors teenaged son that way. They try it again, and they’ll figure it out. It felt good to see the little jerk suffering. And he never made money cutting my grass again!
Now that is 'mean' - the guy stealing the sign will have some severe flesh burning sensations whenever he touches his face, or whatever...think about him having to take a leak before he gets back into his vehicle... :)