You would most likely have a faulty external control (timer/thermostat) or the flow switch is jammed on. Easy way to tell is turn down c/h dial to minimum, if it turns off then faulty external control. If it's still on then turn down the h/w dial to minimum, if it turns off (may not turn off immediately) then the h/w flow switch is jammed.
Ian
July 2009
As a short term solution, if boiler has 2 temperature dials on fascia, turn the one with the radiator symbol down to minimum or off position.
Andy
July 2009
Automatic Answer
We may have found other answers which could help you.