Do you mean 'Lane Departure Warning' the irritating 'tugging on the steering wheel' feature?
Press the button on top of the indicator stalk? Hey, you wondered what that was for didn't you?
I managed to turn mine off - it seems to be permanent.
MMI Home page/Car/Driver Assist/Lane Departure Warning
The manual says it resets itself to be default 'on' after the ignition switch is cycled on/off and this is indeed true. I found that when I started the car and the white steering wheel icon would appear - I'd press the button on the indicator stalk to cancel it....after a couple of weeks it stopped displaying the icon...suits me.