You can close any notebook that you’re no longer actively working in (for example, when a project is completed). Closing a notebook does not delete its content — it merely removes the name of the notebook from your list of recent notebooks.
Note: Closing a notebook does not automatically delete it. For more information, see Delete a notebook in OneNote for Mac.
Do the following:
Click the Show Notebooks icon .
In the list of notebooks, hold the Control key while clicking the name of the notebook you want to close.
On the menu that appears, click Close This Notebook.
Tip: If necessary, you can reopen a closed notebook again at any time.