To erase a ductwork size label from the drawing, use the Remove Label command.
Ribbon: DM HVAC->Ducts-> Remove Label
Pulldown Menu: DM HVAC->2D Drafting->Remove Label
Select the label. This will remove the label from the drawing. It will also prevent the label from being reinserted when you redraw the double-line ductwork.
If you use the normal AutoCAD ERASE command to remove the label, it will be reinserted the next time you redraw the double-line ductwork.