First, you must be logged in as the Admin since only the Admin can remove stock templates from your book entirely. If you do not see Admin in the top-level toolbar, shown in the screenshot below, then you are not logged in as the Admin.
Once you are logged in as the Admin, click on Admin in the top-level toolbar. From there, click on the Stock Element Settings tab in the second-level toolbar.
Click on the Stock Template Library button to display all stock templates.
Uncheck the box next to the template you want removed.
Click on the Save button once you have unchecked all of the templates you want removed.