Adding items to a purchase order via BBP_POC

Hi all

We are currently implementing SRM 5.0 and have came across the following issue. When a user is adding lines to a purchase order via transaction BBP_POC they are allowed to enter any cost centre they wish and they are not restricted by the cost centre attributes within the Org Structure. It does not appear that this can be restricted by authorisation objects. Does anyone know how this restriction can be implemented?

Thanks

Simon