If you receive an unwanted SMS message from Gmail Chat, you can block future messages from the sender. Just reply to the message with the word 'BLOCK'. If you change your mind about that sender, you can send a message with the phrase 'UNBLOCK' to the phone number from which the message was sent (the SMS messages sent from Gmail Chat have their own number associated with the sender's email account).
If you send the command 'STOP,' you'll block all SMS messages from Gmail accounts.