If no one will pick them up then get a bunch of sticky non drying adhesive and pour it all over the pile and see them try to throw bricks that stick to their hands. Contact cement will work great and is not a permanent bond but even would stick to a slick glove. Setup camera's to record plate numbers of vehicles that park to grab bricks so they know who the perpatraitors are. Put a dye that requires a non hard surface to absorb into or just pour a printer cartridge out onto the pile getting in the cracks and they'll wreck their car or rental interior and then the bus or car rental forms for damage can be used to track the people that threw bricks. Cover them with some slime like compound like personal lubricant so they hit the person behind them when they swing back to throw the bricks. Booby trap their ammo is the way to fuck them hard. Hell if their was a substance then cover them in something like skunk scent so they will vomit when they get in the car with the bricks. Last resort cover with acid like substance so they burn themselves in a few seconds of grabbing them like battery acid or such, one cut and it's like razors cutting you.
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