Purchase requisition release strategy problem

Hi All,

We have created a service purchase requisition with release strategy at header level and released it.

We created purchase order with the release PR reference and made the GR/ Service entry.

But now the user is able to unrelease the purchase requisition which has been used for PO creation already.

How the system is allowing to do so?

Is there any control in the system for not allowing the user to do any changes in the PR once it has been used for PO creation?

Thanks & regards

Ram