I was just in the middle of installing a service pack on a Windows Vista machine when Norton 360 thought it would be a great time to run an idle scan. I had turned Norton's autoprotect mode off before starting but I hadn't expected this.
I was then rather frustrated by the lack of an easily accessible "stop scan" option. AVG, for instance, lets you right click on the system tray icon to show a menu that includes a "stop all scans" option. No luck with Norton. I also tried opening the main windows and clicking "Tasks" at the top, but this only showed me options for starting a scan, not a list of scans already running.
After poking around for a bit, I found the answer. Here's what you do:
1) Open up Norton 360. You should see the overview window with four circular buttons labeled "PC Security," etc. In the lower right corner, there's a little icon that looks like a set of vertical lines. Hold your mouse cursor over it and it will show the word "Tasks". Click this icon.
2) You will get a window called Norton Tasks that shows a list of background jobs in the bottom half. Any currently running scan should be highlighted in green.
3) To the left of the running scan entry is a little button with a stop symbol on it. Click this to halt the scan.
That's it! This won't prevent the scan from starting again, but it lets you control the scans once they start.
You can set the idle time delay under settings / administrative settings - this will make Norton wait longer before performing idle tasks (up to 30 minutes).