Can I report a car stolen that is in my name but delinquent by the driver (who is insured)?

A friend asked for my help years back, when I was naive, to re-finance their car in my name. It has caused me a lot of problems over the years, and this year found out there are three years of backed taxes I'm responsible for. They paid some of the taxes, but not all. I've threatened to repossess the car or report it stolen, is that legal?