Is there a way to lock the month transactions so that people cannot post?

Welcome to Community!  I can understand your concern when it comes to changes being made in your books, after you've completed your month's end.  QuickBooks Online provides the option to lock your books, to prevent any changes.  I would be glad to assist!

One of the many valuable features in QuickBooks Online, is the ability to close the books.  This secures your numbers, and anyone who attempts to make changes, will get a warning.  When you 'close the books', and include a password, changes will not be saved without opening the books, and a password will be required.  

Follow these steps to close your books;

1. Open your Settings ⚙, then Accounts and settings

2. Click on the Advanced tab

3. Select Edit ✎ in the Accounting section

4. Turn on Close the books 

5. Enter a Closing date

Note; You shouldn't need to edit any transactions before this date. Your new bookkeeping work will start after this point.

6. Select the Allow changes after viewing a warning and entering a password option from the dropdown menu, if you want to require a password before editing your closed books.

7. Hit Save, then Done.

Please ensure that passwords are between 8-16 characters, and that non-alphanumeric characters aren't being used. For example: . - $#, !)

Feel free to reach out again, with any other questions.  We would be happy to help!