Hi,
First develop a Policy on usage of Company Vehicle/Driver.
The policy should emphasize that Company vehicle/Driver should be used only for official purpose.
Secondly who has to authorize to use Company Vehicle /Driver - please specify.
Thirdly limit the usage of official car whether within the city or out of city.
Subsequently in case of violation you can issue warning letter towards violation of Policy on usage of Company vehicle/Driver.
If you are not maintaining log book for car ( starting kilometer/end kilometre/purpose of travel etc) start maintaining it.