They are held in by two bolts to the wishbone and the ball joint itself is an inteferance fit into the hub, which is then secured with a nut and bolt. you will a 'chisel' type ball joint splitter - looks and operates like a chisel, with a forked end, to remove the ball joint from the hub (scissor type splitters do not fit)
Undo the wishbone to ball joint nuts first (x2), remove the nut and bolt from the hub (you will need a pin punch or similar to drive the bolt out), then the fork of the splitter goes around the shaft of the ball joint, as you hammer the splitter, its wedge profile drives the shaft from the hub - this takes some serious welly, so get a big hammer. This will completely destroy the ball joint so make sure you've got the right one to re-fit.