On App
You can opt out on the app by selecting the menu button in the top right corner (as denoted by 3 horizontal lines)
Choose Profile
Untick "Receive Promotions"
Your customer is now opted out for receiving marketing communications.
On Web
Choose "Order Online" and then Untick "Receive promotions"
Your customer is now opted out for receiving marketing communications.
Via SMS
Your customers can also opt out directly from the marketing SMS's they receive from you. They simply click on the opt out link in the SMS.