Thanks for posting here in the Community, info295.
Yes, you are able to lock a period to prevent any changes to transactions dated earlier than the closing date or previous period.
I've attached some screenshots for your visual reference.
For more information, you can check out this article: Close the books.
I'm also linking an article just in case you need help with changing the closing date password.
Let me know in the comment section on how it goes. I'll keep an eye to your response. Have a great day.