Assuming the crook isn't carrying tools, I attach two large twist ties around the fork blade and skewer handle and pull it good and tight using two pliers. Wrap some tape on the fork to prevent marring the paint. If the skewer handle is slippery, wrap electrical tape on that too for friction. Not perfect but they can't remove it barehanded and hopefully it will discourage the pranksters.

You can also use radiator clamps in similar fashion.

knotty