I've been trying to hide the banner blocks from the admin pages for ages, or at least for an hour and so this is how you do:

Go to blocks menu and press configure for the block you want to set visibility for. Scroll to the end of the page. Press the radio button that says show on all pages except those in the list and then add following

admin/*
admin
node/*/edit

The star / wild card tells Drupal not to show up on sub pages in the admin and any node that is being edited. Pretty handy; to bad it is not on the "Drupal - this is how you set visibility for blocks in Drupal"-page.