Apart from any camera you may have( and check the law in your area, you have to be careful what it's filming). Keep a written diary of all incidents, times, places, names, dates etc, with as much detail as you can. Do not open your door to anyone you don't know. Make your family, friends and visitors aware of the situation and if any of them are due to visit, get them to text/call you to let you know when they are coming. You can also call certain authorities anonymously and explain whats going on. If this is becoming aggressive or frightening definitely inform police so that they have a note of it, but explain to them you don't want the behaviour to escalate. They will advise you what to do. It sounds like you need help/support. Im not so sure it's dementia, some people are just thoroughly nasty pieces of work. Just go about your day as best you can, always have an excuse prepared if you want to get away. Lie if needed, got an appointment, visitor or workmen coming. Be very vague about things.