John,
You have a few ways to keep User from Posting into closed periods:
1. General Ledger Setup (Global Allow From and To Dates)
2. User Setup (Allow From and To) set per user to allow specific Users, like Accounting team, to post to the closing period and future period
The above locks users in all areas from posting to closed periods when posting Purchase, Sales or any related document or transaction.
Hope this helps.
Thanks,
Steve
