Neighbor's nuisance

How do you deal with a flat neighbor's (staying right below) intrusion into one's privacy? Any sound in the kitchen, in the bedroom , in the bathroom,or in the balcony is coincided exactly in accordance with that sound. One is made aware that any loud conversation is being overheard. The construction of the building is such that the kitchen and the bathrooms are built in common ducts. When this nuisance was addressed with a letter to that resident then an allegation of unsound mind as well as a police threat was indicated . This has reached such heights that the family's image is also being tarnished among the co- residents. Seeking a proof in this matter is irrelevant, so it cannot be directly evidenced.. Right now the only option for that family is to relocate their base to another area. Is there any other option? let me know. Thanks.