Then that's going to be a tough one. Just hope that there isn't someone out in the world getting these messages, and say "Who the heck keep sending me these messages, and racking up my charges."
Just in case, send another one with contact information, so you can re-imburse them if this is the case.
JK
It will be tough because you will need to be able to sniff to see if the message is even getting out of your network, and then if it is, then it is anyone's guess after that, since you really have no control after that.
Mark