I am having similar problems with a neighbor's cat. They moved in and he decided he likes my yard better than his. He sits under a holly bush and growls at anyone going in or out of my house including me. I have discussed this with him but he is rather rude and refuses to listen.

So there are frequent spitting matches lots of territorial spraying and occasional minor spats. I still have to face the neighbors who assume that any cat yowling at 3AM is mine.

Seamus is a good deterrent, The neighbor's cat is terrified of him while mine have a live and let live attitude towards their buddy.