I don't know if this works on your car but on mine (a 380SL), I used a strap wrench to hold the pully system to which the fan clutch is connected still. So this may not apply to your car but I think this problem of getting something unbolted off of something that otherwise spins is a generic problem.

You can buy a strap wrench at any hardware store and Sears sells a nice collection of them. You slip it over the thing that moves and spins the fan clutch. You then hold it such that you are forcing the pully system in a direction opposite that which you need to use to get off the fan clutch belt. Strap wrenches are great things, they have proven helpful to me in many applications, from plumbing to cars.