If you have admin permissions in a template collection, you can permanently delete its templates or the template collection itself. This article explains:
Note: Deleting a template or template collection can’t be undone.
Delete a template
To delete a template:
Click your avatar in the bottom-left corner of your tenant.
In Feature Settings, click Template Collections.
Click the template collection and open the template you want to delete.
Click ⓘ in the right-side menu bar.
Click Delete Template. The template will be immediately deleted without a confirmation message.

Delete a template collection
To delete a template collection:
Click your avatar in the bottom-left corner of your tenant.
In Feature Settings, click Template Collections.
Click the template collection you want to delete
Click the gear ⚙ icon in the top-right corner.
Click Delete template collection in the bottom-left corner.
In the pop-up window, click OK.