My first thought is to get that cat trap back and keep trapping them unashamedly...with cameras on and sign a-pointing!
I have a cat, and there is no way I can control where he goes when he's out. It's absurd for someone with four or five cats to assure you that his cats never go in your yard.
Second thought is to borrow a dog for a week or so...that will discourage the cats from coming around!