Hello Ganesh,
It is perhaps a standard set-up. You can arrange to stop it using user-exit. Contact your ABAPer.
Information to your ABAPer
User-exit MM06E005 (F/Exit EXIT_SAPMM06E_012) Code to be written in include ZXM06U43 to prevent changes in PO after SES.
But, you should first confirm from MM space that there is no functional way of doing it. Then only you should use the above information.
Good luck
KJogeswaraRao