what is a persona non grata?
Persona non grata from Latin- an unwelcome person.
This term is usually used in diplomacy, when due to some criminal/political actions a person is prohibited to enter a country. For example if a member of diplomatic mission is caught carrying out espionage missions he can be labeled "persona non grata", forced to leave a country and cannot return.
Persona non grata can also be used in less formal situation, when someone is not welcome in society/community due to his behavior.