Open the customer under Customers, edit the profile and switch Receive SMS? off (tooltip "This user receive SMS?"), then save. Customers can do the same themselves from the Receive SMS? toggle on their account profile page. Once off, the customer is skipped by subscription groups, SMS lists and the main Global groups when composing, and by list sends. For a non-customer contact, edit it under Contacts and turn its Receive SMS? off. Note that direct one-to-one sends from Chat and the abandoned-cart sequence do not consult this flag.