I have too high water pressure (120 psi) on the main line. If we assume the water pressure on is constant, then is it possible to reduce it by reducing the flow using a ball valve? 

The reason I'd like to go this route, is that installing a pressure regulator is a lot of work, and there's not much room where the main line enters the stucco of the house. Turning the pre-existing ball valve, would be a lot easier than installing a regulator.