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:
To unsub, reply with "NO" to 73333
Mobile users who reply as per above will be automatically included in your UnSubscribe list, which you can review under 'Contacts>UnSubscribes'
In your next broadcast, all mobile numbers under this UnUnSubscribe list will automatically be excluded.