To remove a library from your list, select it and click the "Remove Library" button in the main window's toolbar. Note that this will only remove the entry for the library in the list and will not delete any of your files from the disk. If you wish to delete the library's files permanently, go to the Finder and drag the library folder to the trash.

By default, iPhoto Library Manager will warn you when you are about to remove a folder from your list. If desired, you can turn this warning off in the preferences.

See also:

Creating a new library

Adding a library

Duplicating a library

Moving a library

Renaming a library