how to unscrew faucet valve stem when it's stuck?
As Steven says, hit it with some penetrating oil first (WD-40 should work for this). Give it some gentle whacks against a piece of scrap wood in all directions to help break any bonds. Then clamp on to the bottom square part and try to torque the nut on top with a socket. If you still can't get it, you may find it easier to replace the entire thing.