Tell me what 'they have'... they have NOTHING.
These are lowlife squatters sitting in a large area claiming it's theirs. It's not.
Move good people out of the area - then turn off the water and deal with it like a hostage situation. Negotiate them to death until they give up in total frustration.
Then charge them with every crime on the books that might apply. Let the thug's 'mommies and daddies' pay their damn legal bills. This is NOT an army - its a bunch of out of control thugs.
Again, it's a hostage situation. Turn off the water. Have teams used to dealing with hostage situations move in and negotiate things like 'would you like some hamburgers? Pickles with that? How about tomatoes?
Ok - but how do you get in to get the good people out?