1. Go to the start page of the project you want to archive.
2. In the "Overview" tab, click on "Ongoing" or "New" and change the status to "Archived" in the drop-down menu.
3. Click on the button with a checkmark on it to save your setting.
Archiving a course only affects the administrator interface. Participants who have been invited to the course will still have access and 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 mark everyone. 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.