Is it possible to restrict which items a user can place on a PO?

I had a request from the field today. The would like to restrict the items a user can enter on a PO:

Group A: these folks are the "purchasing agents", they can order anything for inventory, resale or manufacture, etc.

Group B: these folks are the "office managers", they can order non-inventoried items, office supplies, equipment not for resale, etc.

I couldn't find anything in Role Maintenance that can limit this. Either you can or you can't enter a PO. Does anyone think it can be done?

Thanks all,

Clare