Is the car jointly owned? Who's name is on the title? Who makes the payments? If it truly is a jointly owned vehicle then I can see where she's coming from. If you make the payments (vehicle, insurance, gas) then no dice.
In the meantime, cut off all her other means of communicating with OM. She'll have to figure out how to pay for all that stuff too so she may back off the car.
If you love somebody, set them free. http://tinyurl.com/2empx2g