Exclude ALL CATEGORIES from search

Hi guys,

I′m using this CODE to exclude PAGES and SHOW ONLY POSTS. >> functions.php

function SearchFilter($querysear) {
if ($querysear->is_search) {
$querysear->set('post_type', 'post');
}
return $querysear;
}

add_filter('pre_get_posts','SearchFilter');

But… I want to EXCLUDE from the results of the SEARCH all the categories, because i JUST WANT to list posts.

Anyone knows HOW?
Thanks!