After parking your car for the night, get a bucket of water and drench the whole vehicle. This process will suck if you hate water spots. However, cats like to rest and sleep on car hoods for the motor's warmth after a drive. The parked car is also security for them since they are off the ground from potential threats. Cats are nasty and absolutely hate getting wet. If your car is wet recurringly, it's no longer a comfortable environment for them to rest. You may have to do this for a couple of nights for them to get the message.
While I don't advocate animal cruelty as an alternative, invest in an air rifle. On good shot on the arse, the cats will remember that this area is not safe to roam around. They have good environmental memory. Just don't fire once they are close to your car
Whatever you do don't feed them antifreeze like I believe someone implied, it's inhumane and they will literally bleed out from the inside and swell up like a blowfish.