1. With a text, you can't withhold the number, so you've got her number. Get lots of peopls to constantly phone it (preferebly men friends of yours).
2. If it's a "pay as you talk" SIM card it may be unregistered, so it's not linked to any person. However ...
3. The phone companies can "ping" any mobile - send it a signal which bounces back and pinpoints the exact location of the phone, like a SatNav. Send this person a message to say (pretend) that his/her phone has been pinged and located, and the messages should at least stop.