To change your course's visibility settings ('Status'), visit your course's page and click on 'Edit course'.
Click on 'Details' at the top - this will take you to the course description page.
In the 'Status' drop-down menu select 'Unlisted' or 'Incomplete' - this will mean that the course will not show up to users browsing courses on Memrise. Don't forget to click 'Save' at the bottom of the screen.
If you set your course status as 'Public', it will show up in the Courses page. Don't forget to click 'Save' at the bottom of the screen.
Please note: even if your course's Status has been set to Incomplete or Unlisted, anyone with the link to the course will still be able to view it and start learning it. For example, if you originally set your course as Public, and then switched it to Unlisted / Incomplete, any user who had started learning the course or had bookmarked it will be able to view it and learn it. The same can happen if you share the link to your Incomplete / Unlisted course with a friend and they forward it to a third person.
Unfortunately there is currently no way to ban someone from a course you created (i.e., prevent them from viewing it or learning it).