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If you are getting unwanted SMS texts from an unknown source you should report it to your Carrier.
From the article Messages: Reporting unwanted messages sent from iMessage found here http://support.apple.com/kb/HT5821.
Note: Report unwanted ("spam") SMS and MMS type messages to your cellular provider. Contact your carrier for additional details.
Look in the Messages conversation thread to determine if a message was sent via iMessage. Messages sent via iMessage are labeled as "iMessage" above the message in the text field. SMS and MMS type messages read "Text Message" instead.
All the best,