Depending on the extent of their lateness, you have the right to ask for an extra fee.
Be aware: In order to do so, you need to write down the time when the vehicle returns on the rental contract.
You can have your vehicle returned in the morning and rent it out again in the afternoon so it is important to ask your renters not to be late in this type of situation.
Last but not least, please note that the renter cannot exceed the return date as the vehicle will not be insured beyond it. The renter will then be liable for all damage in the event of an accident.