Law/rule for recyclers to not accept any cats with obvious grinder marks that could be from removing any kind of ID markings ... see grinding marks, sorry can't take it in for recycling. And to report anyone bringing in such a cat to police on the spot. If thieves knew that would happen when they brought ground off ID cats in, then they wouldn't.
Of course it wouldn't stop thieves from stealing some of them since they probably wouldn't see any engraved ID markings while sawing them off in the dark, and not seeing engraving until getting them home. Guess the thieves would have to find some other crooked way to sell them.