To erase the diffusers that were placed on the electrical drawing using the Coordinate Light Fixtures with Diffusers command, go to
Ribbon: DM Electrical->Devices-> 
Erase Coordination Diffusers
Pulldown Menu: DM Electrical->Light Fixtures->Diffuser Coordination->Erase Coordination Diffusers
Diffusers imported from any HVAC drawing will be erased from the drawing.