The option to delete templates in OneNote 2016 is grayed out. I found a quick solution on the Microsoft Answers site. Copying solution here in case that page ever goes away:
- Go to C:\Users\yourusername\AppData\Roaming\Microsoft\Templates
- Open My Templates.one
- You may be prompted to choose which program to open the file with. Choose OneNote 2016.
- Your templates will appear as pages. Delete the templates that you don’t need. The template will be removed from the “My Templates” section of OneNote.
- This does not remove the template from the “Page Templates” button in the ribbon. The old template will go away once you start to use other templates.
Since your templates now appear as an open notebook in OneNote, you can easily switch to it to delete, edit or create new templates.