Remove the serpentine belt. You can use it to hold the pulley, but you’ll probably have to get a new belt when you’re done.

You’re going to have to wrap the belt around the pulley and vice grip the belt together where the sides meet.

Then rotate the pulley allowing the belt to fold over and back onto the pulley, keep turning it until the free end is short enough to hook on something.

When you turn the fan nut the wrapped belt will tighten on itself and prevent the pulley from turning.