Select & Block the previous participants.
On a typical installation, a teacher will have the right to modify enrollment of a student.
Click on the course. On the left side, you will see the hamburger menu (button with Three horizontal lines) and expand the menu.
Click on Participants. Against each of the participants names, you will see a gear icon. Click the same. You will now be able to see a lot of options that will modify enrollment. I prefer to suspend a user rather than delete them from the course. This will enable you to view the user's activities, submissions subsequently.