do not put a wrench to it
I had the same thing happen to my spindle,
what I did was cut the nut off with a dremel tool cut both flats to major dia.
of spindle threads then take a small chisel have someone hold a block
of wood behind and you tap it until it cracks apart, you might use a wrench
to hold it steady while tapping the groove you cut into nut,
after removing nut you will have to clean out the cross threads,
last and important is to brush out all the threads that are smashed into the spindle threads
I used a fine file to restore the thread profile, order new nut put some oil on
it before screwing onto spindle, it will take some time to do it right.