Use some pentrating oil like Aerokroil or Move-It and let it soak for a while. Smack end of stud a few times with a sharp rap. Wiggle stud back and forth ever so slightly with a 10-12" breaker bar and allen key socket and see if it moves at all. If not, repeat the above. Don't force it and break it off otherwise it will need to be drilled out. As mentioned above, two nuts on the stud would have worked but maybe you can run a thread restorer or die on it? Vise grips are normally a bad idea on threads but if damaged, sometimes it's the only way. If you had a welder, you could weld a nut on it. Use some kind of bar or pipe to lock the studs against the floor to stop the rotor from rotating. Heat is the last option but is usually a good solution.
If "someone" used red loctite on the threads, it'll need heat on it regardless. I used blue loctite on my studs.