Unlike voice, there's no way to block specific SMS messages or sending parties. You can disable SMS completely for one Twilio number, but you cannot selectively reject messages.
If you would like to block all SMS, you can remove the URL from the SMS section of your phone number settings in your Account Portal. To disable incoming SMS on all Twilio phone numbers associated with a Messaging Service, simply uncheck the "Process Inbound Messages" checkbox under Inbound Settings section on the configuration page.
After doing this you will not be charged for, nor will you receive, SMS to your Twilio number(s).
We understand that many customers are worried about abuse, so if your account is ever subject to abuse you can contact us and we will work with you to resolve the issue as soon as possible.