There are a few valves you can use to shut off the water supply to a bathtub. The most obvious of them is the main valve. The main valve controls the whole water supply to your house, and closing it means that none of your faucets or toilets will work. Another option is to shut off the nearest local valve. More often than not, the local valve might be controlling the water supply to more than one system. That means you will not be using the entire bathroom. If your bathtub has a separate valve, it will be present somewhere underneath the tub or above the ceiling works.