Excluding categories from specific keyword searches is not directly possible, however we suggest the following work around:
If for example we are displaying only top three relevant categories in search results, we are able to promote a given category so that it displays within the site search results.
Here we assume that the categories that customer would like to display as suggestion have searched keyword in their title/description.
To promote the categories via Individual product promotions, customer has to search for "KLEVU_CATEGORY" + , this will only display category pages and customer can add 10.0 as boosting score.