Opt-outs in the SMS for NetSuite by Sinch app are managed with an Opt Out SMS checkbox on the associated record.
Opt-Out SMS checkbox
By default, the OPT OUT SMS box is unchecked. If a recipient of a message replies with "STOP", the box will automatically be checked and messaging to the record will be disabled.
When the OPT OUT SMS box is checked, the Conversations button will no longer be shown. Records that have opted out will no longer receive messages sent by Batch SMS or Workflow action.
Note: If the recipient's phone number is present on multiple records, they will still be able to receive messages, so you’ll need to update these records when applicable.