As Steve said, make sure both surfaces of the taper are squeaky clean.
If you have a stubborn one that does not have a hex in the end, I find a little light pressure from a lever will help ( Protecting paintwork etc with a rag if need be )
Turn the nut slowly & it should not spin the ball joint.
If you are just test fitting the ball joints, place a few washers under the nyloc nut.
This will prevent damaging the nyloc & if not over tightened, will allow easier removal of the joint again prior to final assembly
