If a template has no child apps built off it anyone can delete it within the edit templates page. If a template is deprecated but has too many child apps to delete built off it you can create a support ticket here. Tag Marketplace and specify what app you wish to delete and why. In order for an app to be approved for deletion it must be either deprecated or a test application. To prove it is unused even with pallets built off it the state has to be Disabled and no current workflows or rely on the app. Only Admin level Marketplace users can approve this process and they can deny deletion if it seems risky or if it will interrupt any schedules or workflows.