As it's illegal I think you just gotta tell the lads of the dangers to people and damage to property ,and that they will pay for any damage and anyone caught will be instantly dismissed .
Put it in writing and make them sign to say they agree and will abide by company policy, and you need to stick to it if you catch anyone as HSE will prosecute the company as well as the individual, which may affect insurance and future contracts ,
I would turn up for a few hours if you have the lads striking a job you think they may be tempted to bomb and stay there until it's at a level they don't need to bomb , lets them know you got your eye on the ball :blink1: