Two ways (or more), strap wrench on the pulley if you have, or what I have done also is refasten a nut on the fan pulley and then put a 12mm box wrench on it with an extension bar (so it "locks" into place on the frame). Then I undo the stuck nut with another 12mm wrench. Then I undo the 12mm that was "locking" the other in place as it usually comes off a lot easier than the stuck nut, so holding the pulley by hand is enough to undo it.
Sorry, hard to explain, I'm changing the radiator on mine this afternoon, I'll snap pics if you don't get it later today.