Try turning off the Two-Sided option in the AO settings. If you need this setting enabled, you may want to bake two maps, one with it on and another with it off, and blend between them in an image editor.
The only method I know of for completely killing material and polypaint information on a mesh is to select the Flat Color standard material in white, enable Mrgb painting, and use Color>Fill Object on the mesh. That seems to clear everything.