You might have to open the case of the saw and pull out the shaft
connected to the chuck. Grab the end of the shaft in a vise and then
you can put your full body weight on the chuck, without the shaft
turning. If the parts won't fit in a vise you could try grabbing them
with a pair of visegrips held in a vise. Just make sure that nothing
important gets bent or dinged to the point of interfering with use. ie
don't grab gears in the vise. It will destroy them.