Well, since the default Zen Cart template comes with only 6 banner positions, I'm guessing whatever you've done for a 7th is something you've added to your own template.
In that case, the Developer's Toolkit is probably the quickest tool to use for searching your site's files for things you've put in them:
http://www.zen-cart.com/content.php?...-files-to-edit
http://www.zen-cart.com/content.php?...lopers-toolkit