That used to happen on Alfas, the reason being is allegedly the aircon is more efficient if left on all the time but it needs the fan running. On Early up to 2000 Alfas you could turn the system off after completing this complicated procedure.
To quote from the manual:-
Switching compressor control from automatic to manual.
1. ignition key to stop position
2. turn heat control to full right(max)
3. turn fan speed to 0
4. direction knob to full left (face?)
5. turn ignition switch to MAR (the position before engine starts)
6. turn fan speed Auto-0-Auto-0
7. turn ignition key to stop
It should now be set to manual control, to reset it to automatic you need to repeat the same sequence.
I don't know if there is a similar process for your car but maybe.
