Restrict Operational purchase from creating central confirmation

Dear All,

In SRM 7.0 we have  role  /sapsrm/secretary and /sapsrm/op_purchaser.  both of them can create central confirmation.

i want  my opertaional purchase user not to be able to create   confirmation for other PO  , he should be able to create confirmation  for  only his PO.

for central confriamtion i am using /sapsrm/secretary.  Is there any way in role or anything simple that we can restric the central confirmation  for opertaional purchaser.

Thanks

Iftekhar Alam