po amendements can not be possible after gr

Hai experts ,

I want solution for below problem

client wants that , once migo is over for po ( as per fully or partially ) , Its not possible to change the po .

once i raise po and released po . after any wants to change is possible when before gr only.

once gr is over ( partially or completely ) restrict to change po .

As per i know if release indicator function changeable take 1 . but it is effect before gr also to restrict po . i want after gr only its not possible to change po.

can you tell me any bapi or badi or user exits for this one .

i want logic also