There is a margin of error in using dates in that if the format is entered wrong it the prohibition of posting would be incorrect.

To have a "disallow posting to closed period" option would work in theory, as long as it was able to be revoked by an administrator in the system just in case something did have to be posted in a prior period.

I just spoke to our controller for her input as she has worked with numerous accounting systems, and she advised one system utilized check boxes to allow/disallow posting in particular months

ex: there would a be list of all months jan - dec with a box beside each one - boxes to be checked as months are closed/should not be posted in.