Advanced Options -> Exclude results panel
Scroll down on the Advanced Options -> Exclude results panel
Select a taxonomy, then drag the terms (categories) from the left box to the right box to exclude.
Save the options, done.
Posts (or any post type objects) types marked with the excluded categories won't show up on the results list.
By default, selected post type objects which are not associated with any terms will appear in the results list.
If you whish to exclude them as well, then trun OFF the
Show posts/CPM with empty taxonomy terms? option on the
Frontend Search Settings -> Advanced panel:
Category and taxonomy term exclusion will drop posts belonging to at least one of the excluded categories. This way if some posts belong to a certain parent->child1 category and other belong to parent->child2 category and category "child1" is excluded, then the posts from category "child2" will still show up.
Excluding the parent category will however exclude all the posts belonging to the child categories as well, as it's expected.