After removing a student from a Class Notebook, they will no longer have permission to view or edit the notebook's shared content. However, unless you chose to delete their content when removing them, their work will still remain in the Class Notebook.
If you wish to delete their content:
Open Class Notebook in OneNote, delete the whole section group, and sync the notebook.
Re-add the student, then follow the instructions to remove a student. Be sure to check the box Delete student’s content during the process.