There is no "tool" to hold the ball joint pin(s) from spinning that I know of. The best you can do is try to get as much pressure on the ball joint(s) as possible. You'll need a floor jack to place directly under the lower joint, a good (meaning strong--and long) pry bar and an assistant. If you have access to air tools so much the better but if you do use an impact wrench be sure to use sockets meant for an impact -- as it will take a universal socket to reach the nut(s).
Place the car on jack stands under the frame--high enough for the lower control arm to swing down completely. Remove the sway bar link and the lower shock mount. Place a jack under the lower joint and raise it a small amount. Now loosen the lower control arm pivot bolt. Now lift the ball joint/control arm just enough to lift the car's weight from the jack stand. At this point you can attempt the upper ball joint nut but I'd like to point out--if all you are doing is springs there's no need to disconnect the upper joint. But if you do have to -- place the pry bar above the upper control arm and use it to pry the control arm down at the "crotch" of the control arm (this is where the assistant comes in). With pressure applied from above (the pry bar) and below (the jack) you can now attempt to loosen the upper joint (if you really need to) and then the lower joint nut. With the lower joint nut loose and turned mostly off you can SLOWLY lower the jack under the ball joint so the weight is back onto the jack stand. If the ball joint pin is free of the hub assembly as you lower the ball joint jack you can remove the nut and then slowly lower the control arm so it swings down enough to slip the spring out. With the suspension hanging the spring doesn't have much left to go. If the lower joint pin has not come loose from the hub assembly -- leave the jack under the ball joint--with just very slight pressure so when the pin does come loose it doesn't cause everything to "jump". Use a medium size hammer and smack the side of the hub housing where the pin passes through. A couple or three smacks should get the pin to pop free. Have fun. -- Dave