When you remove a student from a Class Notebook, they will no longer have permission to view and edit the notebook's shared content. However, their work will remain in the notebook unless you choose to delete it while removing the student.
If you wish to delete their content:
Open Class Notebook in OneNote and delete the whole section group and then sync the notebook
Or, re-add the student and then follow the instructions to remove a student. Be certain to select the checkmark the box Delete student’s content during the process.