You mean a larger pulley, don't you?
That would make the pump turn more slowly and reduce pressure, wouldn't it? But I don't think you could reduce the pressure enough to matter much that way.
My '83 Toyota Celica has a little white knob on top of the valve cover which adjusts the vacuum signal to a valve on the ps pump which tells the pump whether the engine is turning fast or not -- faster revs gives less ps boost, for highway road feel, lower gives more, for parking, and you just turn the white knob to whatever level you prefer. But that car doesn't really even need ps anyway!