Exclude a category from your Wordpress feed
Excluding Posts from Your Wordpress Feed
That will keep category with ID 5 out of the feed. You can get the IDs by looking in the wp_terms table in your database. With wordpress 2.1, these terms are tags and categories, and while I haven’t tested it with tags, I have seen that it does work with Categories.
Click over for complete instructions. I’ve always wondered how to do this! You may want to check out the original post - some commenters aren’t having much luck with this method.
My solution in the past has always been to set up a new blog in a subfolder - it’s more trouble, but it works flawlessly.
