This article explains how to block a contact from sending instant messages to your mobile phone as text messages.

Currently, Yahoo! users can send text messages to mobiles phone using their Yahoo! Messenger and Yahoo! Mail accounts. These messages can be sent to any mobile phone number, and they appear as normal text message.

When someone sends your mobile phone an SMS message from Yahoo! Messenger or Yahoo! Mail, you should receive a message asking if you would like to accept the text message. To read the message, reply with "Y".

If you do not want to receive any Yahoo! SMS text messages, text block all to 92466.

If you only wish to block messages from a specific Yahoo! ID, text block [username] to 92466, where [username] is the Yahoo! ID of the person you wish to block.