A mobile number could be manually black listed so that it does not receive any messages from you.
This mobile number could be a new or existing contact.
To enable this, please follow the below instructions:
To blacklist a new contact, create a new contact and ensure to tick the "Opt Out" box.
To blacklist an existing contact, edit the contact and ensure to tick the "Opt Out" box.
Please be assured that even if you try to import a contacts file which includes the opted out mobile number accidentally, it would still ignore the contact. However, please be aware that when importing the contacts, ensure not to tick "Allow Opt-Ins" else the opted out contacts in your contact list gets opted in and will start receiving the messages