Very simply: go to administration>elearning>elearning>course
then you'll see a table with the list of courses.
There is a column with an icon 3 people
click the relevant cell and you'll see the list of enrolled students
clicking the red X (the last icon on the right) will remove a single student
selecting many of them and clicking on "delete selected" will remove all the selected
please note from here you can also change the user level in the course.
It's much easier to do it than to explain it...