Remove your front wheel, flip the bike over, insert a long screwdriver from the bottom of your forks and press it against the bottom of the stem shaft then pound the pooh out of it. It may take 20 minutes, but the stem WILL come out. Then your problem is finding a decent new shafted stem where the shaft is not bent on it already.
Oh - make sure you don't have a bolt or other hardware in your stem currently.
I prefer to use a 1/2" socket extension for such pounding activities. You'd be amazed how much crap I've gotten jammed into a steerer, trying to pound the star nut out. If this kid has really bad luck, he'll get a screwdriver blade wedged in there with the stem.
 props to the OP for actually including a photo.