You can place bars on your service to prevent both incoming and outgoing TXT.
This can be done manually through your handset with the following GSM codes.
Incoming TXT
To bar incoming TXT *35*(call bar password)*16# [send]
To unbar incoming TXT #35*(call bar password)*16# [send]
Outgoing TXT
To bar outgoing TXT *33*(call bar password)*16# [send]
To unbar outgoing TXT #33*(call bar password)*16# [send]
You may confirm your call bar password by submitting your request via Contact Us. Click here to access this page
Note: You cannot block individual numbers from TXTing you. If you wish to place an incoming TXT bar it will apply to all incoming TXT messages.