Which of the following actions is prohibited for mechanical fasteners used to support commercial grease ducts?

The use of mechanical fasteners in supporting commercial grease ducts is critical for maintaining safety and compliance with the International Mechanical Code. The correct action that is prohibited is the penetration of duct walls by mechanical fasteners. This is because penetrating the duct walls can compromise the integrity of the duct system, allowing grease and other contaminants to escape or enter the duct, which can lead to fire hazards and diminish the effectiveness of the grease containment.

Additionally, the design of grease ducts requires them to be seamless or airtight to function correctly and prevent grease accumulation that could ignite. Penetrating the walls of the duct can create weak points that could fail during operation, leading to dangerous conditions.

Supporting duct walls, using temporary fasteners, or securing only at the ends are practices that are generally acceptable under certain conditions, provided they are executed according to the code's guidelines. Therefore, option C stands out as the action that must be strictly avoided to ensure safe and effective operation of commercial grease ducts.