You cannot fight these gangs while following the 'rules'. You have to declare war on them. Total war. Announce that anyone living in those buildings will be considered an enemy. Make it known that all enemies found at the location will be neutralized (killed), and that no prisoners will be taken. Nobody will be handed over to ICE, nobody will be jailed and put through the court system. They will be summarily executed on the spot. Sanitize a couple of these buildings in a city, and the gangs will move on to another less aggressive city.