Excluding a category from main page with a plugin

Hi folks.
I am building a plugin that replaces the Dashboard for not-admin users with a ‘Confidential posts’ list: basically, I will use the dashboard as an internal message system.
I have created a category which is displayed in my custom dashboard. Now, the next step is preventing wordpress from displaying messages of the confidential category in pages.
No pages (except in the admin sections) should show the confidential posts.
Now, I think I should use the some filter hook, but I don’t know which one. Can you help me to start this off?
Cheers,
Davide