Be careful on your bashing, that you don't break the back gears or the locking pin.
First thing I'd try is putting it in backgear and don't pull the locking pin; this ought to lock the spindle against rotation. Arrange to have one of the chuck's key holes on top of the chuck. Insert chuck key, and give it a vigorous yank towards you. If you're lucky, that will be sufficient to start the chuck unscrewing.
If you're not lucky, more persuasive measures will be needed, and at this point my initial cautionary sentence about being careful not to break something kicks in. I would be careful of doing anything much more forceful and relying on the back gears to lock the spindle. I don't think Atlas gears are known for their strength. Figure out some other way to keep the spindle from turning. One thing you could do, though it would take a bit of preparation, is clamp a couple of pieces of wood together with two bolts, then bore a hole between them on the dividing line just big enough to go around the spindle. Use the boards and bolts to clamp the spindle in place -- you might have to shave a little off the boards' mating surfaces to get clamping to happen. Then stick a bar between the chuck jaws and belt it with a hammer, as somebody else suggested.
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