Still have a key for your Ex girlfriend’s / boyfriend’s car? Could you borrow a set overnight?
Wait until they go away for a weekend without the car or if they park on the street at night, borrow the vehicle for a couple of hours. Wear a hat and scarf wrapped around the face, or a balaclava, GLOVES at all times, take the car for a tour through half a dozen speed cameras around the area at 15mph over the limit and park exactly back in the same spot before your victim knows the car has moved. Throw your keys down a drain and deny any knowledge of having kept them.
N.B. Please refer to our legal disclaimer, this is an idea for *fantasy only*, we would never suggest that you actually break the law, you could be prosecuted for taking away & driving, speeding, driving without insurance etc etc.