This is a tricky situation.
If you have a hose with a spray gun on it its a great way to get rid of the cat. Cats hate cold water and it won't cause him any real harm but if he gets soaked every time he comes near your place he will remember and stop coming.
At the end of the day a cat is just doing what comes naturally and cannot really be trained unlike dogs. Cats love to do their thing in other peoples gardens/yards if they can. If you demonstrate to the cat he will get soaked every time he comes near he will avoid your place.
Its the owners response that is out of place as they need to be more apologetic and take responsibility for their cat and dog.
I agree with David Z that face to face diplomacy works best and only when its clear that has been fully exhausted would I call the cops. Makes you look like a dick and a cry baby to the other neighbors.
Also if you were trying to kill the cat (maybe you were just joking) I say thats over the top. Just my opinion.
I would be very unhappy if a cat peed in my car but it would be the owner of the cat I would see as responsible. Yes I would chase the cat away, but a death sentence is heavy justice for a misplaced piss
I had a stray who liked to sit on the bonnet of my Acura(Quite a few years back). He was old and had long claws and scratched up my bonnet.
I was pissed off but what can you do with a stray? In the end I just put a blanket over it and we were both happy.
His toilet was seemingly elsewhere as I never had a problem.