I've never worked with an accounting software package before that did not provide the ability to close the A/P module each month once the last invoice has been entered.  The way Blackbaud is now, you have to go into the Configuration menu and close the month to ALL activity in order to close the A/P module and prevent any additional invoices from being posted once you pull financial reports and begin to prepare internal financial statements (usually in Excel) for presentation to finance committees and corporate boards.  The G/L still needs to be open for any last minute JE's, but you do not want your A/P people mistakenly entering an invoice that changes the results for the month once you have printed financial reports.  Honestly, this is accounting 101.