One of the castle nuts just turned with the ball joint stud
this past summer when I was doing some suspension work. It
is possible all you will need to do is put a jack under the
control arm and lift a bit. This will put the weight of the
car back on the ball joint and join it again, holding the
stud securely enough to free the nut.

The cut-off route: You can use a die grinder, grinding one
side of the nut until little is left, and being careful not
to hit the stud threads. A new or recently sharpened chisel,
applied of course in a direction that tends to loosen the
nut, should come in handy, too. I chipped and ground away at
the nut, and even drove a bit on it (though don't be driving
out of the neighborhood), and it ultimately freed.