A simple solution and one I've been using on my cars for a few years now is Epoxy in the screw holes. Fill around the screw, it sets and the screw is set in place. It can be removed if wanted to but it's hard as hell and can't be done in 5-10minutes, they'll soon go on to the next car or be caught in the act, for a plate, they've move straight on, not worth the risk.
The Epoxy resin is clear and looks normal. Just make sure it's the plate you want lol. Mine are personalised plates, once purchased, they remain on. Getting them off is a little tricky and requires some skilled hacksaw work but like I said, easily done in your own time. Be sure to make sure it's the plates you want. No one will be stealing my plates.