Demos in your library cannot be deleted. Instead, they are archived. When you archive a template or demo, it will be removed form your Walnut library and hidden from collaborators. You will then only be able to access it from the archived folder.
How to archive a template or demo:
Enter your Walnut Template or Demo Library. Then hover over a specific demo or template and click on the three dots in the upper right corner and select archive.
This will then remove the template or demo from your library.
You can always view archived templates and demo by filtering your library.
Once you are in this view, you can then restore the demo and have it returned to your main library.