Jack, you've got the easy bit done
. Sod's law says the balljoint will be well rusted to the lower arm
x2. Penetrating oil usually works best with the foresight to douse it the day before the job and again before starting. Personally i go straight for the hot wrench if it doesn't move at all from a few blows. You don't need to get it stupidly hot but the more quickly you can heat it up the better it works- differential expansion an all that

A bit of penetrating oil, and some heat would be my next move, be carefull hitting it, if you put a burr on the top of it it'll be even harder to get out.
x2. Penetrating oil usually works best with the foresight to douse it the day before the job and again before starting. Personally i go straight for the hot wrench if it doesn't move at all from a few blows. You don't need to get it stupidly hot but the more quickly you can heat it up the better it works- differential expansion an all that