RE: Prevent Users from posting invoices in the future
With all due respect, the PSTL tool is supposed to do exactly what you are asking for, 100%. Doc Date Verify does not validate anything regarding periods being open or closed, merely "does this fiscal period exist".Dynamics GP without PSTL does prevent you from posting to a closed fiscal period (GL posting date) and otherwise just warns you if you are using a closed period date.
In your case if you think this was tested and your results are different, I would reach out to your partner for assistance, perhaps see if there is an updated version of PSTL, or something else going on that is interfering. What you describe is literally what this PSTL tool is designed to handle.
From the actual PSTL tool description: "With this installed, when the user keys a document date in Payables Transaction Entry, Receivables Transaction Entry, Invoice Entry, PM Manual Payment Entry, SOP Entry, IV Transaction Entry or Cash Receipts Entry, a check will be performed against the Company Fiscal Period setup to ensure the date is valid. If the document date does not exist within the valid fiscal period setup, a message will pop up for the user that reads “A fiscal period for this date has not been set up.” If a date is keyed for a closed period, the message will read “You cannot post to a date within a closed period. Do you want to continue?” So the user will be alerted with a pop up message in both instances, but may elect to continue on past the message. This will help users to catch any incorrect document dates keyed due to typing errors."