H?ow do I stop the spindel from turning in the mandral?

I have a craftman 19.5 hp 42 in deck model 917258560. The star nub that holds the mower blade in place has been broken off. I can not stop the spindle from turning to get the flange nut off on top off the pulley. There is no pin hole to stop spindle and no flat spot. there is no way to get anything on the shaft to stop it from turning. How do I get the nut off so I can replace the spindle so there is a nub for the mower blade to hold too?