Typically I don't need the airco on to defog my windows. But the airco switches on automatically if you go to the defog setting.
To disable: remove the plastic frame around the gearbox lever.
Put a finger straight up into the gap, in the middle. Slowly move to the right untill you can feel an electric connector. (NOT the one directly behind the switch, mine was blank there)
The connector is where the pointer is on the lowest temperature setting.
Pul the plug. Best done with the airco on the "automatic" setting, so you can check that the airco light switches off. If it doesn't: put the plug back!
Worked well on the European version, 1.5 diesel.