Cut the head off most likely the rest will come out easily.

Can you clamp a vise grips on them?

Take the deck off the rider. It isn't that difficult and then you can work on it. Grinding the bolts is a good idea, but first either put 2 good sides on them or reshape to a smaller size to where you can drive an impact socket on them. The heat from the grinding can help loosen them too.

If that the 9/16" size ones once they corrode and socket slips Good luck. Hopefully the heat didn't ruin the spindles?.

I agree to try the vise grips and BFH after you wooden block the blade from turning. If that don't Grind Bolt heads off. (no grinder Harbor freight has cheap 4 1/2" grinder that will work. Hopefully they will come out of the spindle once the pressure released when you get the blades off.

I never considered taking off the deck. That should give me some leverage. Thanks for all the suggestions!

Take the deck off the rider. (comment from tomplum)
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Good advise.
Decks on Craftsman riders built by AYP are usually easy to remove. This will give you an opportunity to clean the deck and inspect for cracks. Check the legs of the spindles to see if they are broken. Remove the belt from the spindle pulleys and check the condition of the spindle bearings by spinning the blade. Look for cracks in the deck belt.

If your deck is operated by the cable control, you can remove all the lifting attach points - then slide the deck about 1 foot to the right side to remove the cable.