It wouldn’t help with the people tearing it out of the concrete, but if they’re unbolting them, aren’t there locking nuts they could use? Or perhaps, once the nut is tightened over the bolt, the head of the bolt is mangled some way, making it impossible to take the nuts off.
Yes, it wil make it harder for crews to maintain them, but might slow down some thieves.
And then, on a sign that they know is always stolen, a little sting. Catch and arrest even just one thief. and then publicly name and shame and fine them heavily. On the heels of that, offer a reward for those willing to turn in their fellow thieves.