I find when I have a problem with stray cats outside my house that filling my super soaker with neat bleach and firing it at them usually gets rid of them -j/k animal lovers.
Not sure about the US but in the UK you cannot get in trouble for harming a cat, despite what people may think. Cats are actually classed as vermin the same as rats unlike dogs etc. which are classed as pets. If you hit a dog by accident in your car and fail to stop you can be prosecuted, if you hit a cat you cannot.