Once the stop valve has been turned off, the leak still continues with no change, then it means that the leak is not actually on the mains pipeline. This could mean that, if you have a combi boiler and the leak continues, that you have a burst radiator or a burst radiator pipe. If this is the case, the leak will keep going until the radiator, and any linked pipes, are empty and no longer have any water in them. In this situation, the full capacity of the central heating system may spill out. This could amount to the full contents of every radiator.
If your property does not contain a combi boiler, then the water running through your radiators is most likely provided by a feed and expansion cistern. This tank is usually in the loft and it holds all the water that then goes into your radiators. Turning of the mains water supply will stop the tank from filling up with water. This should cause the water from the leak to slow down and stop, but the water that is already within the radiators may still spill out until the leak stops.