If you're using the cover wizard and the book is over 80 pages you have the choice of lettering on spine. Once you see your book cover inside the wizard click the center where the spine is. You can change colors this way too. But look at the top and bottom of spine and see any lettering or text. Double click that then select the text and delete. That should take care of it.
You can increase and change the font size and color there if you wish. Just double click and the box with the options will pop up. Not sure if that is what you wanted or not.