The city of Veii was the first serious opponent of Rome. After three wars it was finally conquered by dictator M. Furius Camillus in 396 bc. Obviously the people were sold as slaves, but the city itself stayed intact.
When Rome itself was sacked by the Gauls, many Roman fled to Veii. When the Roman senate, after the Gauls had left ruined Rome, decided that nobody was allowed to stay in Veii and had the city destroyed; thoroughly.