Is this not what we have station managers for? If you see a kid/group of kids trying to carry a road sign through the turnstyles, you stop them! I suppose that only applies to signage within the metro system itself though. Out in the parking lots is a whole other story. I like the idea of some type of unique bolts, and then maybe even some kind of hacksaw-resistant pole. That could be a tough sell though since it would probably raise costs significantly.
I think Sterling has a good point in that many traditional signs have become so iconic that many think of them as awesome, free home/garage/”man cave” decor. I also think that for whatever reason stealing road signs just doesn’t compute as actual stealing. So maybe the issue is more psychological/sociological?