Once a Stencil has been deleted, you will never be able to restore it.
1. Open the Stencil you want to delete, click on the Ellipsis menu (represented by 3 dots arranged vertically) on top left of the Stencil, and select "Delete stencil" at the dropdown.
2. The Stencil will be deleted. Please note that deleted Stencil can never be restored.