If your automatic sprinkler valve won't shut off, it's because the solenoid has become stuck in the "on" position. It's possible to turn off the valve manually, following a few simple steps. But shutting off the valve by hand won't repair your solenoid, which you will likely have to replace. Shutting off the valve manually will prevent water from wasting, or over-watering your grass to the point that it could become saturated and die.
Locate the irrigation control box, which is usually located outside of the house near a water line.
Open the lid on the box and identify the valve that is running. You can feel the vibrations on the valve that is in the "on" position.
Put on protective work gloves.
Turn the solenoid on the top of the valve clockwise until snug. This will clam down on the diaphragm inside the valve and stop the flow of water. Alternatively, some models have a small tab on the side of the solenoid that you can turn clockwise to stop the water.