Login to your instructor account and follow these steps:

1. Select Manage Sections to the right of your course.
2. Select the blue Students button.
   If not visible, expand your section view by clicking on the section bar or section name.
3. Check the box to the left of the student's name.
4. Select Drop from section from the With selected users drop-down menu. 
5. Select Submit.
6. Select Drop Students.