Rashi on Deuteronomy 13:16/רש"י על דברים י״ג:ט״ז

1הכה תכה THOU SHALT SURELY SMITE [THE INHABITANTS OF THAT CITY] — If you cannot put them to death by the death-penalty prescribed regarding them, kill them in some other way (Sifrei Devarim 94:1; Bava Metzia 31b; cf. Rashi on v. 10).