It should only come on if you turned it on (or left it on).
Are you saying even if you turn it off, it is on when you next start the car?
Where do you have the blowers aiming at? If it is the windscreen this automatically turns the A/C on in some cars, to help demist.
Aircon defaults to "on" when the blowers are set to the windscreen position.
I have them set to windscreen/upper vents to avoid this.