If the mains have been switched off but there is still a leak, and it is coming from a cold pipe and will not stop, then this might be because of your cold-water tank. It is likely that this will be located in your loft. If you have turned the mains supply off this is a great start but, even with this off, there can still be up to 50 gallons of water sitting in the water tank that can potentially leak into your property. To prevent this from happening, and the water spilling through the leak, you need to turn off your gate valve on your cold outlet pipe. This is often positioned somewhere nearby the cold-water tank (and so usually within your loft.) If you are unable to enter your loft for any reason then you can drain the contents of your cold-water tank by turning all of your hot taps on. To make this process even faster you can turn on all of your cold taps as well, but this will only work successfully if the cold taps are not fed by your mains.