1. Open Solution Manager\User Interface\Object Connections.
2. Query for LU Name PurchaseRequisition.
3. Remove DocReferenceObject^ from the service list (do this if multiple items exists in the service list, if not, remove the entire line).
4. Select the same line RMB Refresh cache.
5. Restart the application.

As @ThiliniK mentioned, you can do this by disabling the Document Attachment Panel in Purchase requisiton window. 

If it only certain documents/types then custom event / event action could handle it.

or

if it is because you want users to do it on pr line level then event for header could have error message to tell user to do it on line level.