Dear Abby: How do I handle an intrusive neighbor?

Dear Abby: I have a next-door neighbor who is retired and takes care of his wife, who has dementia. He’s a nice enough person, but he has no personal boundaries. He is intrusive and knocks on my door while I’m working two to three days per week. My work area is close to the door, and if I ignore the knock, he just knocks again. I am on the phone most of the day for my job, and his incessant knocking is disruptive.