Block Requisition approval


I’m trying to block requisition approval to all users except for one person, who will approve all the requisitions.

What I have done is set bo.Req.BuildNextDispatcher in service security to block all users expect for one. This kind of works. When the non authorized tried to approve the req, they get this error message.

The problem I’m having is, once they click ok, the system will still allow them to approve the req. Is there something else I need to do make this work?