Currently, there is no way to delete a title that has been created in NetGalley. In most cases, archiving a book is the best option. When you archive a title, it is no longer active in the catalog, can no longer receive requests, and does not count against your NetGalley title limit.
If there is a scenario where archiving a title is not appropriate (for example, an embargoed book) please contact concierge[at]netgalley.com with this information and we'll get back to you as soon as we can.