Entry denial means to deny a person access to Arstotzka. It is marked with a red entry denied stamp on a passport, temporary visa slip or passport seizure slip. The stamp also has the current in-game date (or November 23rd, 1982 in the latest beta) and the Ministry of Admission seal.
Care must be taken when denying entry. Once the inspector has stamped a passport with the entry denial stamp, no matter how many times the inspector later stamps the passport with the entry approval stamp, the entrant will be denied.