Single edge razor blades inserted in the corroplast along the bottom edge. When the Lefty goes to pull out your sign he leaves his fingerprints on your lawn. Literally. Alternatively bury wire in the grass that is hooked to the metal frame on one end and an electric fence charger on the other end.
fence charger is a great one. we raise meat goats and to keep them in we have to use a 12 joule charger box
when I sometimes touch it...knocks me to the ground and I do not feel right for a few days..hehehe