furnace gas burner will not shut off

If it shuts down when you disconnect the power to it the problem is not with the gas valve. You say you disconnected the thermostat wires and it is still firing; then it is getting power from somewhere. Disconnect the wires on the gas valve. It should stop at this point, provided the gas valve is not stuck in the open position. If it does stop, check the low voltage wires for shorts. You most likely will find some wires burnt and melted togeather and feeding power to the valve. Let me know what you find out and if I can assist you more