How can I prevent iOS 7 from downloading to iOS 6 devices?
Apple uses the following hostnames for distributing OTA updates:


mesu.apple.com
appldnld.apple.com


Blocking access to these hostnames should prevent iOS from downloading the updates, resulting in one of the following errors, based on whether the device has knowledge of the update or not before the access is blocked:


Software Update Failed: An error occurred downloading iOS 7.
Software Update Unavailable: Software Update is not available at this time. Try again later.
Unable to Check for Update: An error occurred while checking for a software update.






iOS devices use the following XML file to find the relevant update and download file:


http://mesu.apple.com/assets/com_apple_MobileAsset_SoftwareUpdate/com_apple_MobileAsset_SoftwareUpdate.xml


Blocking access just to this file should be sufficient providing that the devices have no knowledge of the update prior to your blocking.