I have no experience with ADHD. (Then again, that is also what I said about autism and then I turned out to have known two slightly autistic people for about four decades. I’ve learned a lot from them about autism since then.)

People with ADHD can fall victim to a phenomenon called “limerence”. They can obsess and fantasize so much about you that in their experience, you are a part of their life and vice versa.

I don’t know whether telling them in a stern voice that you’re a stranger and want nothing to do with them actually works.

About one third of people with ADHD, however, apparently also have NPD.

That’s a different ball park. Cut off all contact. Say absolutely nothing. Give no explanation, no apology, nothing. Say nothing to other people who mention the person either. (You may want to look into “hoovering” for more about this. Hoovering is making attempts to draw you in again.)

https://www.sciencedirect.com/science/article/abs/pii/S0022395624002383