On the left-side of the screen under Payment Processing, select Email, then Send Payment. On the Templates page, you can click on the "X" icon next to the email you'd like to delete. This icon has been highlighted below in red.
Once you've clicked the icon, confirm that you really want to delete the template. Clicking "Yes, Delete" will remove the template from your list of available templates. Clicking "No, do not delete" will keep the template on your list.
Note (1): If you decide that you need to make the template again, you can always add a new template.
Note (2): The Stock Templates may not be deleted.