Somewhere there should be a pressure relief valve. Do you have a vent pipe to the outdoors, a little copper pipe? If so, trace this back and look for a valve. These are really safety valves, but can be operated manually to lower the pressure.
I would go with ConfusedJohn's suggestion as the safety valve method will work, but in my experience they dont like to be disturbed and can start leaking once they have been touched. better to be safe with the Bleeding the rad suggestion. than sorry with having to replace a safety valve that wont shut off again.