Please refer below link to setup opt-out preferences of your customers based on their SMS response (Please note you need to first setup lookup configuration for this object to make auto opt out to work based upon incoming SMS received-http://www.screen-magic.com/incoming-sms-related-list/)-:
Once you have Opt Out preference of customer mapped in "SMS Opt Out" field, you can filter opted out members easily.
To avoid sending messages to opted out members -:
- If you are sending messages with send SMS button, then set "Send SMS To Opt Out Members" to "No"
- If you are sending auto messages with workflow rule, then add rule criteria as -SMS Opt Out" boolean is equal to "False"
I hope this helps. Free free to write us on firstname.lastname@example.org if you have any questions further.