Whom did King Joash order stoned in the courtyard of the temple?

Answer: Zechariah son of Jehoiada

Zechariah son of Jehoiada rebuked the people; Joash forgot Jehoiada's kindness to him and had Zechariah stoned.

“Then the Spirit of God came upon Zechariah son of Jehoiada the priest, who stood up before the people and said to them, “This is what God says: ‘Why do you transgress the commandments of the LORD so that you cannot prosper? Because you have forsaken the LORD, He has forsaken you.’””— 2 Chronicles 24:20 (BSB)