"N'importe qui" would be like "it doesn't matter who" or "anyone" in the sense of randomness. The usage would be like: "Anyone can win the lottery as long as you buy a ticket." "Pas un(une)" would be "no one" or "not anyone" in the sense of absence. This usage would be like: "I went to the conference room at 10:00, but no one was there. The meeting had been moved to 11:00."