Big name retailers have begun to implement large scale WiFi monitoring systems to track all smartphones in their stores. By monitoring WiFi traffic they can identify unique devices by the MAC address associated with your phone. Stores which subscribe to this service can determine if customers are repeat visitors, how long they spend in the store, and what areas of the store they visit.
Euclid, a major provider of this service, does claim that no sensitive information is collected. However, this may not be reassuring to all privacy advocates.
Nothing can fully prevent your device being tracked without disabling WiFi completely. For best privacy measures it is recommend that WiFi be disabled when you leave your home and office.
Below is the link to opt-out of the service:
To disable WiFi on Android, the icon highlighted below can be tapped to toggle WiFi on and off:
To disable WiFi on iOS, under the setting panel, the icon be switched on and off as shown below: