I prefer to by anonymous 

Today I have used rental car with promotional code to get to work, while my car is on maintenance. Whole ride cost me less than 1 euro. Ride was about 45min long and 10km.

Basically, company did not really earn anything from my ride. How you gonna protect your business from that kind of people ?

P.S. Frankly I prefer taxy over car-sharing. Same price, but you dont have to do anything.