Mother in law driving vehicle without permission, what to do?

My unlicensed and uninsured mother in law is driving a vehicle in my wifes name and we do not want her driving until it is in her or someone else's name. We are unsure of how to legally protect ourselves besides to report the car as stolen. I was thinking maybe sending a certified letter to her explaining that while the vehicle is in her possession we do not give her permission to drive the vehicle nor do we give her permission to renew the registration of the vehicle and if she is caught driving the vehicle by us or by the police we will report it as stolen. This seems a bit harsh but the risk my family is taking on when she is driving it is so much, I don't know what to do. I was thinking about some how disabling the vehicle but would much rather do something a litle more legally binding to help protect my family. Any ideas?