Ok, since everyone else is weighing in on this:
My top 8 recommendations (in the order I'd do them) would be:
1. Talk to the owner if you can locate him/her. Explain (calmly) that you don't want to kill the cat, but if it continues to sit on your car you may be forced to do so.
2. Get a car cover. Let's face it, you probably want one anyway, and this is the perfect excuse (for your significant other) to get a cover because covers are a LOT cheaper than a new top. Make sure it's a light colored one - it will stay cooler and the cat won't be as tempted to lay on it. A bonus is that it will protect the car from bird droppings as well...
3. Spray the cat with a garden hose. Make an exaggerated run for the hose any time you see the cat near your car. This way opening the door to your house should be enough to scare it off. After 2-3 times the cat will probably find another place to sit.
4. There have to be things you can buy (Rhino piss, something like that) that cats can't stand that you can spray around your car (not necessarily on it) to keep the cat away.
5. Make friends with the cat. Then advertise it for adoption on Craigslist. (hee hee)
6. Trap the cat, drive 50 miles away and release it (or bring it to an animal shelter). My preference would be to let it loose - shelters are overcrowded and after a certain length of time they often are forced to kill the animals.
7. Get a weak spring driven BB gun. You want something that won't hurt the cat if you accidently hit it. Then aim high (so as not to hit the cat or your Boxster) and when you see the cat near your car, fire to miss it. Remember to aim high and to the side - BBs from weak spring BB guns will drop 3-5 feet for the first 100 feet. Make sure you aim so you don't hit cars, people, or neighbors houses. After the first few times you do this you don't even have to have BBs in the gun - the Pfffftt sound alone will make the cat run...
8. Get your own cat and train it to guard you Boxster. You'll know it's doing it's job when you hear a lot of hissing...
NOT recommended:
Anti-freeze - you REALLY shouldn't do this - it's one extremely horrible way to kill a cat (or any other creature that drinks it). A slow, horrible, painful death. Tuna isn't necessary, cats will lap it up regardless because it (apparently) tastes sweet and very good to them. This is also probably illegal in most places.
Bazooka - for obvious reasons.