You cannot delete a template, but you can leave it unpublished so it’s not visible to anyone. You can modify the Published state as follows. Open the template using the modification link that was emailed to you. Click the link to modify the form, and to untick the Published box. Once this has been done, the link leads to nowhere, and nobody can see the form any more. However, you will be able to access the form again through the modification link in your email.