You can delete an individual biblio item by going to its Edit form: click on the reference title, then on the Edit tab. At the very bottom of this form, there is a red DELETE button.
You can also delete many biblio items at once. To do this select 'Administration' from the main menu and click on 'Manage Content'. Use the FILTER tools to select items which are "status is published" and "type is Biblio". You will then get a list of all your biblio items in the table below. Click on the table header to sort your items either by title (useful when you are deleting duplicates), by author (i.e., the person who entered the reference), or time added. Then select the boxes for the items you want to delete in the lefthand column, select Delete from the Update options menu, and then click on the UPDATE button.
CAVEAT: When you make an update in the Content table, this will affect only the items selected on the current page; i. e., when you navigate to another page in the Content list your previous selection will be forgotten! In order to get all your items on one page, you can change the Max rows value under General Settings above the Filter controls.