difficulty removing lower suspension ball joint 21mm nut

[charliedog]

Have tried everything bar cutting it off with an angle grinder. If anyone knows how to get this nut off 21mm I would be delighted to know how. The allen key socket in the bottom of the shank of the joint has worn round, the joint goes round and round, it appears the nut is rusted to the shank..........?????? fed up, I have spent days and all sorts of methods to release this nut
Any help would be appreciated

[GPW]

Grind it from one side so it's almost ground to the threads - but leave about 1 or 2mm away.
You'll now find the nut is much easier to undo and will probably snap open if it's really stuck - and come off regardless.

I did this recently with a steering balljoint nut so I know it works, the heat and the thinness of the metal left weaken the grip considerably.
The same method can also be used to get an inner race from a front hub - grind it close and then whack it with a hammer and it will crack and slid off.

[PhillipM]

Jack right next to it on the lower wishbone if you still have the uprights and springs on - the weight of the car pressing down will lock the taper into the wishbone and let you work the nut off.