If you're using Twitter for SMS to follow others, you can choose to unfollow the user by texting UNFOLLOW username to your carrier's short code. (Example: UNFOLLOW buttonsmc)
If you don't want to unfollow the user completely, but only wish to stop receiving their updates via SMS, you can text LEAVE username, or OFF username to the same short code. (Example: OFF buttonsmc or LEAVE buttonsmc)
To learn more about Twitter for SMS and SMS text commands, please see this article.
Our Troubleshooting pages for Following issues have answers to common problems.