Here is what I recently came across and may help you, it may not.
I took my arms off a old blaster, The frame is junk. But I came into the same situation when taking them off and apart. they want to spin, so you cant tighten or loosen up. the ball joints.
I have some ideas, I have not done this yet. and likely wont need to.
However. i can share what i was thinking, it may help you.
I was gonna take a die grinder, a dremel w grinding disc should work also.
Gring a slot into them and use a big flat head screw driver to try and hold them. But my feeling was it would not work well.
My 2nd idea was to drill a whole and thread tap, so i could put a bolt into it to hold it from spinning with a wrench, why I tried to tighten or remove the ball joint nut.
Other than trying a big pair of vise grips that lock to try and bench it, to hold it, and try turning.
Those were what i was thinking of trying and doing.
Sporty