Has this escalated into stalking, OP? It sounds as though it might have, or at the very least be bordering on that. This is something that might bear consideration.
Your question is one, unfortunately, that it's impossible for anyone to answer other than the harasser themselves. They behave this way for a variety of reasons. They might want to make their presence known. Or they may be in denial, and when someone says the word 'no' they determinedly hear precisely the opposite. (This, incidentally, is a massive red flag). Sometimes, this can be motivated by what's known as an erotomanic delusion that the other person loves them. It can be impossible to reason with this.
Otherwise, it can simply be an element of control. A person who has previously had control over you is most persistent - or possibly dangerous - when they fear they are losing that control. For this reason, albeit you're convinced they represent no threat of harm to you, you still need to exercise caution.
It's imperative that you have no further engagement with this person. Don't answer the phone or the door. Instal a ring doorbell and ensure your home security is up to scratch.
It might be worthwhile reading the police advice online as to what you should do if you are being stalked. Whatever the situation, however, the more you engage, the more you reward their behaviour and the more they will persist.
I'm sorry this is happening to you. I've been stalked: twice. It's a horrible, invasive experience and no one deserves to go through this.