An action Plan -
1. find the feckers
2. prod them with sticks
3. repeat 2 till you get bored

It may be a little difficult to come up with a plan as it must depend on the type of paint used, unless they left the tin behind you would have to work that out first.
Getting the stuff of could be by Heat, chemicals, high pressure jetting or scraping - none if them sound like they would do the stones much good. Good luck to whoever has to deal with this.