Hi Chris,
Thanks for reaching out!
Access to a course that's hosted on an LMS is entirely controlled by the LMS. You can check with your LMS support team to see if they have a feature that will lock a course once it is marked as completed.
In Storyline 360, you can restrict access to a test in your course by creating a True/False variable that you can set to True once a learner reaches the end of your course. By referencing this variable, you can then control what happens when a learner attempts to go back to your test (Ex. redirecting them to a different section in your course).
If you can share your LMS details here, I’m sure one of our talented community members who has used that LMS before might have some suggestions for you.