You might have several AO sources. Raise also the shadow bias of the lights, and in the Scene AO settings. For me, usually Shadow Bias settings of 0.7 (up to 1.5 for severe cases) work fine. Although at 1.5 there isn't much AO left; Thatfor I guess that you changed a not-relevant value, because at 5.0 (Poser Unit) you shouldn't have any AO left.