That's the way to remove an unbroken chuck. However, if it's broken, the OP might well have problems in securing an allen key in its jaws. In that situation, it may be necessary to attach a clamp of some sort to the chuck and then hit that with a hammer.
Silly question (but I've made the mistake myself!!!) - I presume you're sure that you're hitting it in the right direction? If so, your only real hope is probably to leave some pentrating oil to do its work for a few hours and then try again. Any attempt to 'jamb' the works so that you can apply continuous force to the stilsons will almost certainly do irrepairable damage.
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