I used to destroy peoples gardens when i was a kid, i remember the owner telling me not to over and over but it was where i played, and it didnt register in my head that i was not meant to play there, i think you should definitly put some poison ivy down, they wont want to play there any more, you could also plant some nettles around the bricks the keep knocking over, just try and make the garden the opposite to where you would like to play if you were a kid.