In OneNote, click "Insert" tab -> "Pages" group -> "Page Templates" command -> Open "Templates" Pane -> on the bottom of the pane -> "Create new template" -> click "Save current page as a template".
OneNote will create a new template, and save it into "My Templates" group.
In the "Templates" pane, expand the "My Templates" group, right click on the template which you want to delete. On the popup right click menu, click "Delete" item, OneNote will delete the template. If "My Template" group is empty, it will disappear.