You can try getting the car cleaned professionally to get rid of the cat smell or move the car to another parking spot for a while (week or so) and this might cause a break in its routine.
If you catch the cat in the act and you could try drenching it with water. My dog used to scratch his bum on cat bumpers and set car alarms off - that sort of thing would scare the brute senseless.
I have to warn you that I have two cats and they constantly take the pi55 out of me but, thankfully, neither spray. Strip wallpaper and destroy woodwork are their specialities.
Have fun - remember that the average cat is 1/20th of your size and not as intelligent. :-)