Not related to BI platform search imo: we faced the same issue and we run our AO workbooks on the Netweaver backend, not via BI. SO i think above suggestions will not help you
After investigating our case, i think it is an autorization gap where users are allowed to see all documents (workbooks) in AO by default. This is handled by object S_RS_AO and we plan to solve it as follows:
* give "full" auth on workbooks in the namespace WMU* (so they can save workbooks starting with that in the name in their "My Documents"), but also restricted on "$USER" so you can see/use only your own workbooks
* give "execute" authorizaion on workbooks in the namespace WMC* which are the centrally maintained workbooks in the roles.
An annoying gap in this solution is that they still will be able to search for WMC* workbooks that are in roles that they don't have, but the actual execution (ie refreshing) of these workbooks is then blocked via correct S_SR_COMP en S_RS_AUTH checks.
Hope this helps...