This is nothing to do with the Police but I think you know that.
If they call again either don't answer the door or do so and just say clearly you're not interested in anything they have to say and to please leave you alone.
My Step-Grandfather was an absolute asshole who abandoned several families before getting with my widowed single Mum Grandmother in the 1960s and having 2 children with her. He was abusive to my Gran, my Dad and his other step-child and both of his biological kids with my Gran. He died in 1986.
A few years back she was really distressed to receive a letter from 2 of his previous children making enquiries about him. Presumably they'd looked up old records or been on Ancestry.com and as my Grandmother has lived in the same house for 70 years, they found her.
Their half-siblings by my Gran wanted nothing to do with it so I just wrote to the newly discovered siblings saying yes, he was their Dad but no-one in the family wanted to meet up or give any more information.
I felt for them to be honest, it must be hard to not know your heritage or what happened or if you have more relatives you don't know about but politely making it clear you don't want contact should do the trick.
I'm sorry this has brought up bad memories for you.