You can include hyperlinks for users to click to unsubscribe, or prompt the user to reply with an SMS keyword.
For hyperlink unsubscribe see https://support.commzgate.com/hc/en-us/articles/360000157201-Using-Unsubscribe-Hyperlinks-in-your-broadcast-message
For SMS unsubscribe with keyword, the recipient must first exist in your Contacts modules. (either via bulk upload or manual entry)
Then within the message that you are sending out, include this in your message content:
Basic Plan users and higher
To unsub, reply with "NO" to 73333
Go Plan users
To unsub, reply with "NO" to 93841957
Mobile users who unsubscribe will be automatically included in your UnSubscribe list, which you can review under 'Contacts>UnSubscribes'
In your next broadcast, all mobile numbers under this UnSubscribe list will automatically be excluded.