The SMS Mailer application has a blacklist module. It is disabled by default but you can activate it by going to Settings > General and enabling "Use BlackList".
When people send STOP to one of your mailings, they will automatically be added to the blacklist, even if you did not enable the option.
When creating a mailing, the application will check if you have the Blacklist enabled or not, and prepare the mailing accordingly.
This allows you to disable the blacklist for important non-commercial messages.