Some times one can use a cold chisel to break the hold of the nut.
The nut will have to replaced, so try to loosen the hold on the stud by placing the chisel at an angle and giving it a few sharp blows with a big hammer.
The idea is to jar the nut off by rotatating it in the anticlockwise direction, the nut will get stressed and will become larger internally and should become loose enough to remove.
Be sure to get a big enough bite that no nut material will be cut off .
Be careful not to take too big a bite that threads on the stud will be damaged.