Over heating of central heating system?

I have a frost stat in the garage (where my boiler is) and a normal room stat (set to 20degrees). When the frost stat kicks in the manual room stat seems to become useless as the boiler contiues to heat the radiators up untill piping hot . I turn the room stat down but nothing happens (the room stat clicks at over 30degrees). I eventually have to turn the mains switch off for an hour or so then wait for the radiator temperature to come down. Then switch it on again, untill the same thing happens. It makes no difference if the heating is either on or off?
Any clues please?