When using the mib_amb_occlusion shader to perform an occlusion pass, you can choose to exclude one or more objects from the occlusion calculations.
In the object's transform node, expand the mental ray section. In the Flags section, disable Derive From Maya and turn on Hide.
For example, assuming that $targetTransform is your transform, set the miLabel as follows:
addAttr -at short -longName miLabel -defaultValue <your value> $targetTransform;
For example, if you set your id_inclexcl value to 8, then only transforms with a miLabel of 8 are included in the occlusion render.
If you set your id_inclexcl value to -8, however, then all transforms with a miLabel of 8 are excluded from the occlusion render.
See the mental ray for Maya shaders guide for more information about the mib_amb_occlusion node.