I would turn off your phone/text message system for awhile; make arrangements to contact other friends other ways and/or I'd just ignore, as in totally as if she's invisible, anything coming from her. Any response at all is what she's looking for so no response as if you don't care would probably extinguish it completely. Think of it like spam in email. If you turn off your phone for a week, she'll get tired and your message center full, etc. Another thing you might do is contact your carrier and ask them what, if anything, they can do to block messages coming from that number. Even if your phone doesn't have a feature, I imagine they do.

The only other "legal" thing you can do is see a lawyer and get a restraining order. I had a boss and his ex-girlfriend wouldn't leave him alone (and at one time she had worked with us) and he had to do that. Every time she called after that he would turn it over to the lawyer, etc. and it would be followed up on.