I have a similar problem with my MX5, except that it is my own cat and he is mostly white and practically lives on the roof!
The best way to deal with cats is water - a shotgun is likely to damage the car too much and could result in severe injury to yourself if the cat's owner is anything like me;-)
The fun way is with a supersoaker type water gun or you can get hose attachments that have a motion sensor so that cats get sprayed when the enter the garden. This may not be an option in an area with water shortages though.
Scents like lavender, rosemary or citronella can work as long as you don't mind it yourself. My own cat won't even walk through our herb bed in the garden and that has rosemary, lavender and sage in it - but other cats aren't bothered at all.
One of the sticky rollers you get for clothes is the best way to clear the hairs.