My elderly parents have a neighbour who is making life very difficult for them - it's got so bad that they're afraid to do anything in their garden or leave the house for an extended period in case he visits when they're not there. When they do go out, they look for excuses not to return home.
I suspect the root cause of him being like this is that he wants them to sell their property to him, when they've made it clear that they aren't going anywhere.
I've pleaded with them to get their friend, who is a Justice of the Peace, to go and have a polite chat with him but they don't like conflict (which I get - who does?!) and feel he'll just come 'round eventually. I don't think he will! I think, left alone, things will only get worse.