In order to remove an existing reference database from the current PC, please mark (click on) the corresponding line in the Reference Database Library dialog. Afterwards, press the "Delete" button at the bottom of the dialog. Once you have acknowledged the warning message, you will be asked if you would also like to "physically" remove the corresponding index files. If you answer "No", you can reactivate the corresponding reference database later on using the "Add" command. Once you press "OK", the name of the currently marked reference database will be removed from the Reference Database Library list.