I hate to call police pigs. But some of them really are pigs.
The fastest way to stop all this, I think, would be to sue them for millions or tens of millions of dollars.
Government, like everybody else, hates to pay out money. When they are forced to pay to the brink of bankruptcy, mayors or whoever are the bosses, will make 1000% sure that police brutality will never ever happen again. Not because of concern for the public, but because of money.
People are cheap. They know of no morals. But when you take money out of their pockets, they learn.