In my case, the stuck nut came loose after the first cut. Cutoff wheel in a die grinder as close as I dared get to the threads, then popped it open with a cold chisel. After that it unscrewed just fine. I think I did have to deburr a thread or two with a triangular rat tail file just to cleanit up for he new nut to turn freely.
With luck, though, you won't have to learn this kind of surgery yet.
Merritt Island, FL
When a person asks you for advice, they don't want advice. They want corroboration.