I've just had the exact same problem. It was an absolute nightmare. first I tried WD40, I ended up using a whole can (one of the smaller ones) on the stem, over a period of about a week, to absolutely no effect whatsoever. I then tried plus gas, soaking it for ages and still to no effect, other than a bit of rusty liquid coming out of the bottom of the steerer tube. Much wrangling later, and after a failed attempt with boiling water I decided to give a bench vice a go. Though the stem was OK, and I would have preferred to keep it in good condition I didn't mind sacrificing it in order to get it out of the bike, allowing me to fit the stem I wanted on the bike...
I turned the bike upside down, with a bench vice clamping the horizontal of the stem. Removing the front wheel, and front brake, I placed a spare hub and skewer in the dropouts to keep the forks spaced, and then used a timber length between the forks to work against the stem. It took absolutely ages, and was quite a frightening experience, but eventually I managed to get the stem out! I'm sure this would get pretty much any stuck stem out, just make sure to put a hub in the forks, and go slow and steady to work it loose!