You can not actually delete a replacement car. You can only set the car as inactive, so it won’t be visible in the planning anymore. The rental history of the vehicle will always remain available.
You can set a vehicle to the inactive status by going to the tab "car fleet" - "courtesy car". Click on the vehicle. Then click "↦ Show more details" and finally click
In the "car fleet" - "courtesy car", there is a filter available on the left. Below you can see "Only active vehicles." To see inactive vehicles, simply uncheck this box.
You can set a vehicle back to the active status by going to the tab "car fleet" - "courtesy car".
Untick the box 
