My guess is the next passengers will report it and the car will just drive back to it's home to be cleaned. If it's a big problem you could have the offender re

My guess is the next passengers will report it and the car will just drive back to it's home to be cleaned. If it's a big problem you could have the offender report it for a reduced cleaning fee or have the internal cameras recognize it (or show an internal picture of the car to people before they are picked up). Those are just off the top of my head but it's far from an intractable problem.