Service PO limits

Hi Gurus,

I want to stop the user to change the limits in service PO after the service PO has been released and entry of service entry sheet.

e.g.

I have service PO with overall limit of 100K and it has been released.

Now, user entered the service entry sheet with reference to this PO of 20K.

Now if user go to me23N and will enter the 200K as limit of service PO,then it will change and also will not reset the release for it.

I want to stop this change of limit in service PO after it has been released.

How can i configure the system to do this?

Please guide me.

Thanks