If you are going to ditch the blade anyway, can't you lock the blade in a vice and then remove the nut? The blade shouldn't turn on the shaft. Alternatively, get an oxy torch and cut the nut off. Just use a welding tip and when you have the nut cherry red turn up the oxygen and you should be able to blow the nut away without damaging the thread on the shaft. Have done it plenty of times with frozen nuts when all else failed.
