Archiving a course only affects the administrator interface. Participants who have been invited to the course will still have access. You will even be able to invite new participants! The only difference is that the course won't be visible under "Active" courses but instead under "Archived" courses.
If you wish for your participants to no longer have access to your course, go to Participants and check the box of all participants. Then click on "Change expiration" and choose for example yesterday's date (this way their access will end immediately). Lastly, archive the project. Now, neither you nor the participants will see the course. Read more about how to change participants' access here.
Go to the start page of the project you want to archive.
In the "Overview" tab, click on "Ongoing" or "New" and choose "Archived" in the drop-down menu.
Click on the checkmark to save.