Success at last!
OK, I was finally able to get that bolt out. I did it by finding an answer to my original question of how to keep the spindle from rotating. And finding that was just dumb luck when I was looking over some tools on display at Home Depot and spotted a wide-jaw and large pair of vise grips for 14 bucks. With that clamped onto the spindle (photo attached) and blocked against the inside of the mower deck, it provided enough resistance to keep the spindle from turning with the impact wrench hammering on the bolt. It might be that I could have used a breaker bar with a 3 or 4 foot pipe over the handle, but since I now have this nice impact wrench, that's what I chose to do. I'm now glad I didn't resort to cutting the hex head off, because once the blade was loose, it still took a lot of torque all the way to unscrew the bolt using a socket wrench. Looks like the bolt was rusted in. One thing for sure, from now on I'll be using anti-seize compound when installing blade bolts, as suggested above.