If you look in the 'custom.php' file for your theme, you'll see that there is a line like this:
add_filter(array('Display', 'Item', 'Dublin Core', 'Title'), 'show_untitled_items');
This is the code that's causing the [Untitled] display.
You can simply remove that filter to allow titles to be blank, but I wouldn't recommend it. Omeka generally uses an item's title as the text for the link to the item itself, so if you allow the title to be blank, you can create blank, un-clickable links.
You have a few options for dealing with the titles. You can create a new filter of your own (see the documentation) that display the genus as the title if there is no title.
You could also make sure that you set all the titles the way you want them in your CSV file before importing.