Three ways.
1. Customer can simply click on the unsubscribe link provided in the footer of each sms they receive.
2. A customer can be unsubscribed in the store by the staff on the Clover station.
3. A customer can be unsubscribed from the UMarket->CUSTOMERS-> any customer listing.
Either way, the customer will cease to receive your promotions.
Please also see different UMarket Marketing Settings.