Does the lack of a written employee-monitoring policy prevent us from disciplining?

Q: We have five drivers. We told them verbally when we hired them that we may use GPS from time to time to track their activities. This is not in writing, not in the handbook and there is nothing they have signed acknowledging this information. However, we used a GPS device on a driver this past week and found out that he parks his truck sometimes for as long as three hours and clocks this time as working time. What can we do? Can we discipline him for this letting him know that we are aware that he is stealing time? Can we terminate him for this? – Kerri, South Carolina