Screw holding wash impellar won't loosen

I have new parts to repair this dishwasher. Trying to loosen the screw holding the wash impellar(the impellar has 6 flats on it to keep it from turning) the motor shaft the screw is screwed into keeps turning but the wash impellar won’t turn because of a wrench on the flats. Is there anyway to prevent (lock the motor shaft?) the motor shaft from turning, so the screw will loosen?
Thanks for your help

You can try a pair of vice grips on the old impeller to get the screw off. Once screw is off, I use a sharp wood chisel to bust the old one off. If you sheer off the screw, you can epoxy the new impeller on

You can use vise grips to hold impellar