Loosen the upper control-arm ball-joint bolt (do not remove yet). Use a hammer to smack the control-arm flat spot in order to break the ball-joint stud loose. Once loose, you can then remove the ball-joint nut. Don't let the steering knuckle hang by the brake line; use a wire or string to hold the knuckle up. Keep the bolts and such, as you will need the hardware for reinstallation.