The Black codes evolved into Jim Crow, brutal laws that governed what Black folk could or could not do. Between the police, the KKK and the white citizens' councils, there were lots of official and unofficial groups policing Black folks' movement.
White folk could harass Black people themselves, or they could escalate up the chain of violence, either the official one, or the unofficial one. "I'll call the police on you!" Or "I'll call the Klan!" Or "I'll call my husband and his brother on you!"