Keep certain categories of posts from front page list

I'm on vacation using a laptop that doesn't have all my old WP custom code, and need to stop using Front Page Manager plugin to restrict which categories of posts show up in the center column on the front page. Something about it is not working all of a sudden, so I want to go back to the method I used to use, custom PHP code that would go into the index.php. Something like "if is_home, don't display posts from categories 15 and 20".

Obviously, not in plain English, but PHP code. Anybody have that handy? I don't want to bug Flynn with this now since he's busy with ThemeFrame. Juggledad? Off the top of your head?

By the way, I'm using enhanced recent posts widget to do this in the left column widget.