My kittens also love to tease the chickens! They are trained not to hurt them, but they love to bat at tail feathers or terrify the OEG bantam. I even got a picture of it:
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What I would do is when the cat tries to enter your yard rub your feet on the ground repeatedly while glaring at them and maybe even growl. This is cat language for "get away, my territory". It works fine on my cats when I want to keep them out of the shed or something. You could also chase them off with a shovel or something every time they come near and hopefully they will get the message soon.