Please follow these steps:
1) If the course is free then make the course private.
OR
2) You can set maximum student for the course, so that new users cannot take the course, refer:
http://prntscr.com/a56pvh
3) The product that is connected with the course, make its stock as 0. So new users cannot purchase it.
OR
4) You can hide courses completely, go to wp-admin->wplms->course manager->hide courses from directory, refer:
http://prntscr.com/a56r9s.
This will hide courses for all. However the students who are already enrolled in that course can see the course in my courses tab, refer:
http://prntscr.com/a56sdu