have you tried to unscrew it from the fork threads yet?? and if it's shouldered down onto that top nut, the Nut ITSELF might force the stem loose...
if that doesn't work, get out the pipe wrench and go all animal on it.. twisting will free it quicker than all the hammering in the world, usually.

and then, there's a keyhole hack saw... cut it just above the top nut, then start wearing some blisters into your hand sawing it in two places until you see steel chips.......then hit the sides, and then twist it.

another thing to try.. find a Cheapo Steel Stem to clamp onto it.. one that you can sacrifice.... then use it for leverage to TWIST until the extender rotates........

and if the top nut will come off separately.. Shock the extender Sideways with a Dead Blow hammer of 3 to 4 lb. weight.. IE: A BIG Soft Face Plastic Hammer. this will loosen the bonds that are defeating you, so far.