They make a special spanner wrench that fits into the 2 holes in the front of the water pump pulley to hold it so you can get the big nut lose. If you do not have access to one, you can try this trick that works for me.
Back out 2 of the 13 mm bolts that go through the pulley into the pump and put a large flat bar between them and hold the bar while you turn the nut. Use the bar to lock the pulley in place and remove the fan clutch. Big screwdriver will work, but needs to be pretty flat to get a good bite. The nut is a regular right hand thread.
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