A couple of methods I've used...one is to put the wrench on the fan clutch nut, then whack it with a hammer (Make sure you hit CLOCKWISE- the threads are reverse from "normal"). Try this 3-4 times and it might work.
If that doesn't work, you can try wedging a thin, long bladed screwdriver in between 2 of the bolt heads that hold the pulley to the water pump- the fan clutch wrench can actually be used to help keep the screwdriver blade from slipping off. You should be able to get enough leverage with one of these methods- though I've had some stuck SOOO tight I didn't think they were ever gonna come loose. If you get one of these, try soaking with PB Blaster or some other quality solvent overnight, then go at it again.
Bret.