Just don't talk to them any more. Leave them alone, and don't take their calls. Return their mail. If you want something more decisive, you could go to court and ask for a restraining order, but you'll need a good reason.
You can get what they call a "Cease & Desist Order." I, unfortunately, had to do it to my own mother. She was a fine woman until she got up into her mid 70s, and then she became paranoid and irrational, and I had the police banging at my door. They had done a check on me and knew I was not a threat to my mother (I had been her caregiver) and told me this was not uncommon for parents to harass one of their children when they got older. It broke my heart, but my health came first, and I went to a lawyer and had it done. Later, I found out my poor mom had dementia and was eventually put into a nursing home. She was given a good medication that slowed down the process of dementia (does not cure it), and at least I had a few bitter-sweet months with my mother. We got along just fine because she recognized me as her daughter, but of course she was dying. I thank God every day I had that chance before my mom passed away.