What you really need to do is disable the Email app, if you are going to use K-9. Go to settings then application manager, choose the All tab at the top of the screen. Find the Email app and choose disable. If disable isn't there, choose Uninstall Updates and then the Disable button should appear. The way you currently have it set up, you have two copies of each email on the device and that is a waste of resources.
If, for some reason, you must do it the way you asked, go into the email app, tap menu > settings and then choose each email account that you have set up and disable notifications. It is the first check box on the settings screen for each account. You have to do each and every account to get rid of notifications completely unless you disable the app as described above.
Hope this helps,
BigRedGonzo