Family and neighbors trying to defame by using my nickname

My name is Manmohan. My mother call me Manu in home while my grandmother used to call me Mastu. But now all my relative calling me mastu ( which is embarrassing name). They are calling me intentionally to defame me. Can I send them legal notice stop this. They are saying my real name is Mastu , telling that your grandmother give you this name, we have right to say Mastu.