If the joint that you are talking about is the taper fit between the
ball joint and the steering arm then the method I have used
1. undo the nut a couple of turns
2. pre-load the taper joint by placing a jack underneath the pin of
the balljoint and jacking up lightly against the weight of the car
3. smack two hammer heads simultanaeously against opposite sides of
the steering arm, radially to the ball joint pin.
This should "spring" the taper free, and the pre-load provided by the
jack will then release the interference fit.