If you're talking about the ball joint on the bottom of the hub then mine were the same last week.
Take off the brake calipers so you can get to the bolt head and get a socket on it. You might need a bar to get better torque but try and twist it a few times to break the rust and spray WD40 on it too. should get easier then it hammer out.
Failing that undo the three 13mm bolts that hold it to the wishbone.