To delete a staging site just go to
WordPress Live Site > WP Staging > Sites / Start
There you find a list of available staging sites and a delete button on every list element:
Sometimes it happens that even after deleting all staging sites you still have some WP Staging tables in the database. This can happen if a cloning process has been interrupted and did not go to the end successfully.
To learn how to delete those tables as well follow this link.