Because OTRS is designed to be a fully "auditable" system, items such as Agents, Customers, Groups and Queues cannot be deleted. However, you can effectively hide unwanted items from day to day operation by marking them as "Invalid" in edit view.
The only items that can be deleted by default are FAQ articles. To delete tickets you'll need to enable the delete button in SysConfig, or create a Junk queue that deletes contents on a regular basis.
Follow the directions below to mark un-deletable items as "invalid".