I had that happen once. I used an old set of vise grips, really cranked them onto the stud with about 1/2" of space between the steering knuckle and the vise grips, taped the handles together with tape so they wouldn't pop open, and used a ball joint fork between the spindle and the vise grips to separate the stud from the spindle. Put the nut back on upside down until it's flush with the end of the stud, if you can, to help keep the stud from mushrooming if you do more hammering on the bottom. It's harder to get it back out if it's mushroomed.