If you hear of a city where people serve other gods, what should you do, according to Dt 13:12-17?

Kill everyone (even the cattle) in that city and burn it down.

If thou shalt hear say in one of thy cities, which the LORD thy God hath given thee to dwell there, saying,

Certain men, the children of Belial, are gone out from among you, and have withdrawn the inhabitants of their city, saying, Let us go and serve other gods, which ye have not known;

Then shalt thou enquire, and make search, and ask diligently; and, behold, if it be truth, and the thing certain, that such abomination is wrought among you;

Thou shalt surely smite the inhabitants of that city with the edge of the sword, destroying it utterly, and all that is therein, and the cattle thereof, with the edge of the sword.

And thou shalt gather all the spoil of it into the midst of the street thereof, and shalt burn with fire the city, and all the spoil thereof every whit, for the LORD thy God: and it shall be an heap for ever; it shall not be built again.

And there shall cleave nought of the cursed thing to thine hand: that the LORD may turn from the fierceness of his anger, and shew thee mercy, and have compassion upon thee, and multiply thee, as he hath sworn unto thy fathers. Deuteronomy 13:12-17