Get a really big adjustable wrench and a large hammer (1.5-2 lbs). Place the wrench on the retaining nut/shaft of the fan clutch at the 1:00 position and strike it towards the 11:00 position with the hammer. (Drag from the belt and accessories should provide some aid, so don't remove the belt until after you get it loose. After 2-3 blows it should break free and allow you turn it off with the wrench.
There is a special tool for removing the fan clutch that *some* AutoZones allow you to rent, but the above procedure works just as well.
Word of advice, once you get the fan loose, be careful not to damage the radiator fins with the fan, it will still be in the way and the assembly has some weight to it.