The term, persona non grata, is a powerful one in international diplomacy. Meaning “unwelcome person,” it's a declaration that someone is effectively banned from a country.

A 1980 memo prepared by the Department of Justice for President Jimmy Carter in consideration of actions to be taken against Iran outlines the legal authority for ousting foreign agents, tracing it back to the earliest days of the American republic.

A State may declare a foreign diplomat as an undesirable person in accordance with Article 9 of the UN Convention on Diplomatic Relations in Vienna.

The host country is not obliged to motivate its decision, and the accused must leave the state within 48 hours.

The diplomat is usually recalled by his country's leadership. There is no provision in the in Vienna UN Convention concerning the ban's removal. 

The diplomat declared as undesirable person may return to the country in which he was on mission only as a ordinary person. The term "persona non grata" is currently used in international diplomatic language, but its origins have never been fully investigated.