You can make a figure/actor/prop "Invisible to raytracing", which will stop it creating any AO. However, if you are using raytrace shadows it will no longer create any direct shadows either, which can be a problem.

The "Invisible to raytracing" option is in the body/actor/prop Properties palette.

(edited to add:) also, invisible to raytracing means it won't show in such things as Reflection, Refraction..

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)