How it is deactivated is described here.
Steering wheel keypad and combined instrument panel.
Cruise control - On/Off.
Standby mode ceases and the stored speed is resumed.
Standby mode
Activate and adjust the speed.
Selected speed (GREY = Standby mode).
Cruise control active - WHITE symbol (GREY = Standby mode).
Cruise control is switched off with the steering wheel button (1) or by switching off the engine - the set/stored speed is thus cleared and cannot then be resumed with the 
button.