Cut off everything down to the top o the fork, or headset nut, or as close as you feel comfortable going with a hacksaw, then you can finish with a file, until you're down to the inserted diameter. That will allow the headset to come off and the fork to be removed.

I don' know i you own a hacksaw, but can tell you that many bike shop "pros" are th wrong people for the job, because they'll either refuse it, charge you too much, or botch it anyway.

You want an old fashioned, dirty fingernails mechanic, willing to take in a challenge because he likes interesting job and is used to doing work beyond simply hanging new components on a frame. If there' a co-op in town, that may be the right place to go. Or, if you cn free the fork yourself, you might be be to mail it to someone like Yellow Jersey in Wisc. to have the stub removed at a reasonable price. Email Andy in advance for a quote, and tell him Francis from Chain-L sent you.

BTW- before going the saw route you might be able to dodge a bullet using the locknut as a screw jack to forse the stem out. Use a 2x4 to keep the fork from turning, and back the headset nut up against the stem, then crank on it and see if it will jack the stem up. This may mar the top of the nut, or may strip it, or may even strip the fork if the threads are already damaged (unseen), but it's a shot if you don't go crazy.