You cannot delete the ready-made report templates that we provide as they are protected. But you can delete custom report templates, accidentally or otherwise. 

Any reports produced so far from the custom report template remain intact. They continue to show the name of the report template they were created from, even though that report template no long exists. 

However, any schedules set up for the report template will be removed, including those of other team members. Therefore, before we delete a custom report template, we ask you to check that no-one else in your team needs the report template any more.