Make it mandatory for all of your police officers to buy private malpractice insurance.
Buying mandatory malpractice insurance would be very difficult for an LEO to purchase, if insurance companies know that they'll be at increased financial risk for taking those specific individuals on as clients.
Who would want to be a cop, if their insurance premiums were a quarter of their annual salary?
Edit: The upward pressure on malpractice premiums for all LEOs, caused by bad cops costing big settlements, would create internal pressure to run bad cops out of the department. Keep them premiums low. Earn discounts for actual serve and protect behaviors.