Block the option to edit, delete and add documents from previous months

• go to Settings > Account settings, 
• Then click on Configuration > Invoices and Documents, 
• In the Invoice lock created prior to the date of  section, select an option convenient for you.

• Confirm the changes by clicking Save at the bottom of the page.

If your accountant has access to your account, only they can change this date to an earlier one, giving them control over when they can 'close the month'

Locking invoices created prior to the date of and expenses

Locking invoices created prior to the date of applies by default to both income and expense documents.

If you want the lock to not apply to expenses, check the Exclude expenses checkbox visible next to the Invoice lock created prior to the date of field and save the changes.

This way, expenses added under the account will be excluded from the lock.