I dont know what you can do stop to it, mindless idiots are a plenty & its not always kids. To me theres only 2 solutions; i. move them to a museum (which I doubt ppl would want to happen) ii. 24 hr trigger happy armed guard (which will be too expensive)
I'm not sure education would help, cos it'd highlight the stones to more defacing idiots who didn't know they were there in the 1st place.
Summat needs doing, but theres certainly no easy answer.