Assign the cars and the other vehicle types to different vehicle classes. Then allow only the parking vehicle class to park.
For the details of what you set, I would have to look into Vissim, but the concept of partitioning vehicle types into vehicle classes and then assigning different behavior to the different classes is pretty much standard for every network object, so it should definitely work for parking decisions as well. Probably, there is an attribute that the decision is only valid for certain vehicle classes.