How to delete the iOS7 install package on iPhone?
I did not have this issue and so didn't have to deal with it. But there is a solution posted on the Apple Discussion Forums that you could try. If it doesn't work, you could follow up there (and subscribe to that thread to get updates). Make sure you have a backup (along with Time Machine backups of your previous backup and this one).

Here are the instructions from that post (with better formatting and some typo fixes):


  Part 2. Delete iOS 7 firmware downloaded automatically:
  Method A:
  If you have backup in iTunes, recover it. (I didn't test this method, so it's on your own risk.)
  
  Method B: 
  
  
  Open Settings > General > Software Update
  Tap Install Now.
  There will be a prompt that says "Verifying update...".
  Wait until this prompt disappears, there will be a spinning wheel on a black background. Immediately press and hold both of the power and home buttons at the same time.
  DO NOT release the power and home buttons for about 10 seconds.
  Your device will reboot. It's done. The automatically downloaded firmware will have been deleted.