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Just adding into this.The way we all tighten or loosen the center nut is to put a screwdriver through one of the holes in the pulley to lock the pulley from spinning.
With that being said, I'm sure that's not what they do at the dealer, there is a tool for everything.
I don't remember 100% but is there a hole in the front of the shaft for either an allen wrench or 3/8 ratchet? If so, that would hold the shaft while turning the nut with a box end wrench.
Sounds like you need a new PS pump...complete with pulley. Based on your description and from my own experience the pulley is stripped on the shaft. The nut simply holds the pulley in place and does nothing to tighten it on the shaft. The shaft has serpentine groves on it, if I remember and this holds the pulley in place on the shaft.
Our confusion is that I don't believe we've ever run across this before. The pulley should be difficult to remove from the shaft once the nut is removed. In your case, the pulley is spinning freely on the shaft. No amount of tightening on the nut is going to make it stop spinning.
Notice in this pic, the grooved/spline area past the threads... this is what locks the pulley in and keeps it from spinning. The nut simply holds it in place. Possibly something internal in your pump froze and caused the pulley to strip on the shaft. Either way... a new pump is going to be needed.
