You cannot delete an ebook from your account. if you want to remove a title from sale, you have to deactivate it. Please follow these instructions in this case:

1) Log into your ebookbase account on https://www.mobipocket.com/ebookbase
2) Click on the link "Browse all ebooks" or search for the book you want to remove with the search box.
3) Click on the title you want in the results list.
4) You will be redirected to the details page of your ebook.
5) Uncheck the box "Activate for Retailers".
6) Click on "Save and close book information" button at the bottom of the page.