Even though it's handy for cooling the car quickly, the Owner's Manual P198 recommends limiting it's use due to possibility of stale air. If you turn the recirculation off and choose demist, it gets turned back on and stays on when returning to the other vents.

To stop it coming on automatically, turn the temperature control 1 notch clockwise from maximum cold setting (fully anti clockwise). It then doesn't activate unless you press the recirculation button. This isn't in the Manual.