Who actually gets prosecuted? The city is just an area on a map. Is it the mayor? The city council? Or does it devolve down to a money grab from the citizens unfortunate to live within the city boundaries? I haven’t a clue. It’s not like you can send a city to prison.
You start at the top of city government. When their replacement takes over — Deputy Mayor — ask them if they will immediately reverse the sanctuary policies. If they refuse, you arrest them as well. Keep going until you get to the one city council member who as interim mayor will comply. Charge them with obstruction of justice, accessory after the fact for any crimes committed by illegals, and conspiracy, and sedition.
Once you have City Hall announcing compliance, you go to the police commissioner and ask them same question. Arrest and ask the replacement, repeat as many times as necessary.