To remove a library from your list, select it then either click the “-“ button or select “Remove Library” from the File menu. 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 really do want to move the library to the trash, you can hold down the option and shift keys, and the “Remove Library” command will change to “Delete Library” instead. This will both remove the library from the library list and move the library to the trash for you. Note that you’ll still need to empty the trash via the Finder or Dock in order to delete the library permanently.
By default, PowerPhotos will warn you when you are about to remove a library from your list. If desired, you can turn this warning off in the preferences.