Can you 'lock' a course when it's complete?

Alas, there's currently no way to lock content in Rise. I've seen other posts asking for that feature. You can submit a feature request (https://articulate.com/support/contact/feature-request ), but it may or may not got implemented.

In the meantime, you might want to consider creating a duplicate of each course as a backup. You could even change the title in the dupe so it starts with "BACK-UP" and/or has the version date in the title. Then you could go back to the dupe to copy blocks (via creating block templates) if needed. 

Another option would be to save all the blocks in a given lesson as one big block template, which could later be used to recreate the lesson, if needed. When you know it's no longer needed, delete that block template. 

Here's more about block templates: https://articulate.com/support/article/Rise-Creating-Sharing-and-Reusing-Block-Templates.