Hi Srini,
Did you try creating a validation rule at Document Header level to check if the user is allowed to post in the closed period.?
You can use the field Posting Date BKPF-BUDAT from the document (being posted) header to check if the same is within the allowed current period date range. If not, the user would face an error message. You can achieve the check by getting a simple exit composed by an ABAPer. You can create a set of users (for whom this check to be performed) in GS01. And this set can be a part of your prerequisite.
You will have to define prerequisite, Message in OB28 and have the exit as check.
Regards,
Ravi Kumar
