Is there a way to lock an accounting period once reconciled?

Yes, there is a way to lock an accounting period once reconciled in QuickBooks Online (QBO), accounting. I'd be happy to help you with that. 

You can set a closing date to lock the period, which will prevent any entries from being posted after it has been closed. When you're ready to close out the year, close your books to prevent unwanted changes before filing your taxes. This locks your books so no one can edit your accounting data before the closing date.

Before closing your books, it is important to review your accounts and make sure that everything looks good. You should enter any outstanding invoices, expenses, and payments and reconcile your accounts up to your closing date. Additionally, it is important to review your inventory quantities.

Then after ensuring, you can proceed to close your books. To do that, here's how:

1. Go to Settings and then select Accounts and settings. 
2. Click on the Advanced tab and select Edit in the Accounting section.
3. Turn on the Close the books switch.
4. Choose a closing date that allows you enough time to complete your bookkeeping work before this date.
5.  If you want to require a password before editing your closed books, select the Allow changes after viewing a warning and entering a password option from the drop-down menu. 
6. Hit Save and then Done.

For more information on how to close your books in QuickBooks Online (QBO), you can refer to this article: Close your books in QuickBooks Online

If you need to change the closing date or forgot the password, you can check this article for more detailed steps: Edit your closed books in QuickBooks Online

Please let me know if you have further questions about closing books in QBO. I'm here to help. Have a good one.

I am more interested in closing a month once reconciled. I understand closing the year, but do not see a means to close a month so entries are not posted after the month is reconciled.  

Renee