Not to rebuild it as a city

This mitzvah forbids rebuilding an ir hanidachat — a city destroyed for its turn to idol worship. Once such a city has been condemned and burned, it must remain desolate forever, serving as an eternal reminder of the gravity of communal idolatry. Rebuilding it would dishonor the sanctification of Hashem’s Name that was achieved through its destruction.

Commentary & Classical Explanation:

• Rambam (Hilchot Avodat Kochavim 4:9): Once destroyed, the city is never to be rebuilt, nor can its ruins be made into another settlement.
• Sefer HaChinuch (Mitzvah 465): The permanence of its ruin warns all future generations of the consequences of idolatry.
• Talmud (Sanhedrin 112a): Even converting its land to farmland is prohibited; it must remain a ruin forever.