The pilot reported that, after landing on an elevated helicopter deck and reducing the rotor RPM, he signaled for the brush cutting ground crew leader to approach the helicopter to collect additional chainsaw bars that were being dropped off. He added that, the crew leader indicated that two of the three chainsaw bars were not the correct bars needed, and then threw an incorrect bar away from the helicopter. Subsequently, the bar impacted a main rotor blade, which resulted in substantial damage. The pilot reported that the brush cutting crew had been briefed before operations on gear security, rotor awareness, and safe practices when working around the helicopter. The crew leader admitted to a lapse in judgment brought on by stress and frustration. The pilot reported that there were no preaccident mechanical failures or malfunctions with the helicopter that would have precluded normal operation.