If you want to exclude posts from certain categories you can add the following to your index.php page for the given theme that you are using.
Before the block of code that checks for posts with have_posts just add the following:
This line of code, along with the surrounding code is:
To determine the category ID’s you can log into the WordPress administration area and look at the links for the category ID’s. The – in front of the category ID’s above specifies that those category id’s won’t be shown.