You've started, so you've got to carry on now
The allen sockets in the end are a waste of time. The problem is holding the balljoint stud once you get it moving, even those with spanner flats on the back can be difficult to hold.
What I use is a small set of stillsons that are narrow enough to wedge in between the balljoint and the ARB or mounting bracket, so they bite into the waist of the balljoint stud to stop it rotating. A ratchet will whip the rusted nuts off, they can be tight, but I've never had to resort to the grinder yet.
Footprint pliers also work well, or at a pinch, mole grips - but whatever you use, they have to be narrow enough to grip onto the balljoint stud.
Some heat on the the nut also helps. Those small, high temperature butane torches, beloved of chefs, work surprisingly well on individual nuts.