There are moonshine trucks running around the area. They're quite easy to spot visually and they also appear as red vehicles in your minimap. When you spot one while on foot or driving, shoot its tires first to stop it on its tracks and aim for its fuel tank.
Take note that the driver and his companion are both armed so you have to make sure you won’t be caught off guard. Also, prepare to leave right away since the civilians nearby may phone the police. You don't need to do this always since their only good for inflicting damage to the racket. If you already fully damaged it, then risking your life taking one done is practically useless.
After inflicting the amount of damage to bring down the racket to zero income, report back to Nicki Burke.