回複：今天發現有人橫穿我家後院，

I would suggest that you filed a complaint to the company. If they repeat, you have an option to sue them for trespassing, in which you can recover any property damages or at least a nominal compensation ($1) if nothing is damaged.

Your sign "trespasser will be shot" does not make any sense. This only time that you can use deadly force is when the trespasser is threatening your life. Otherwise, under no circumstances can you use deadly force to protect your house or property. If somebody did get shot and die you will be liable for murder. However, you can use reasonable force such as push by hand to get them off your property.