I can think of two possible solutions.
First, if you never plan on moving those cars, then you could always put a small break in the track between the crossing and the cars. It could be a very small gap as long as the track doesn't connect to the crossing.
Secondly, you could place a short piece of invisible track just a hair above the visible track and place the cars on the invisible track. Make sure the invisible track connects to nothing.
Of course, neither one of these solutions will work if you plan to move the cars.