A big Auth-hold transaction can mitigate this problem. An Auth-hold of $501 will prevent prepaid cards from being used. It also forces debit card users to have $501 available in their checking account. This should be enough to stop amateur thieves.
Professional thieves are a different matter. Traditional rental agencies have cameras at the rental counter, and get a copy of a driver's license. Building LoJack into the carkit won't work because it is too easy to block a GPS signal with tin foil. The only realistic way to stop professional thieves is to get driver's license numbers and review driving records for all members (which is what ZipCar does).
Professional thieves are a different matter. Traditional rental agencies have cameras at the rental counter, and get a copy of a driver's license. Building LoJack into the carkit won't work because it is too easy to block a GPS signal with tin foil. The only realistic way to stop professional thieves is to get driver's license numbers and review driving records for all members (which is what ZipCar does).