I have the same problem. They make wrenches for bicycle cone bearings that are thin and you may be able to get one that will wedge on. My plan is to get it off and file some flats on the arbor so I can get it loose if it happens again and when I get a new saw make sure it has an arbor lock or do the same with it. My plan now is to use a socket on a drill and spin it. If it is spun clockwise and then quickly stopped the inertia might be enough to move it. Once it moves it should let go, I wish you luck, I'm not really into taking the saw apart yet. Design malfunction.