The best thing to do is to contact your local Animal Control and Protection or Humane Society. They will come and catch the cats and remove them from your property. Trying to do something yourself could result in legal problems for you. Killing the cats could be construed as cruelty to animals and some areas have huge fines and even imprisonment for this type of thing. I understand how frustrated you are as people who don't have pets of their own don't want to have other people's uncared for animals in their yard. Another possibility would be to purchase a motion activated sprinkler called a Scarecrow. If an animal walks by the sprinkler system it will shoot water at them and scare them off which is a humane way of getting these cats out of your yard. You can get the Scarecrow at Home and Garden Supply centers or order them online. One link you can get them at is
Good luck. I sincerely XXXXX XXXXX can get these kitties off of your property safely.