My guess is that you have a defective water valve. Take the cover off of your wall mounted humidifier control and disconnect one of the two wires that are connected to the control. If the water valve does not shut off, then the valve is probably defective. The only other possibility I can think of is, your unit may have a latching relay that has locked the valve open. Not being an expert in this area, I don't have enough knowledge to know if your system has a relay installed. You could trace the wires from the valve to their souce and disconnect one there, but I suspect this may not be practical. Perhaps one of our more experienced colleagues may have another solution.