The "electric fence" idea of running a hotwire to the battery positive terminal might not be that difficult to do. The hardest part would be insulating the star mount from the grill itself - maybe using ceramic, plastic or thick rubber gasket. This would not put a drain on the battery until somebody got zapped.

Or have you thought about permanently welding the star to its mount?

After twice getting it ripped off, now is the time for serious action. It almost makes you want to sharpen the edges of the ring to a razor's edge, doesnt it? Or maybe a skilled jewelry craftsman can cut a groove in the ring for installing actual razor blade edges from cheap disposable razors, glued in place.