Social retribution, mostly. If you disrespect me, then you disrespect my friends and my family, and my associates and comrades and such. Without money to er, grease the gears so to speak, people had to form much stronger relationships, and build reputation with each other. And generally people lived with each other for most of their lives, without moving far away, so they could establish that trust.
Even in that system, sociopaths can bullshit to damage someone's reputation, to turn their friends and family against them, but they can't do anything that will protect themselves from scrutiny. There may be bands of fools following sociopaths competing over how big assholes they can be, but they ultimately turn on each other, and communities can separate them out, and y'know... fight them off.