WordPress search filters

functions.php

//to exclude a custom post type
function remove_portfolio_search() {
	global $wp_post_types;
	$wp_post_types['dt_portfolio']->exclude_from_search = true;
}
add_filter('pre_get_posts','remove_portfolio_search');


//To exclude some specific post
add_filter( 'pre_get_posts', 'cp_filter_search' );
function cp_filter_search( $query ) {
	if ( $query->is_search && !is_admin() )
	$query->set( 'post__not_in', array(68,11) ); //68,11 are ids of the posts to exclude
	
	return $query;
}


//To exclude a post category
add_filter( 'pre_get_posts', 'cp_filter_search' );
function cp_filter_search( $query ) {
	if ( $query->is_search && !is_admin() )
	$query->set( 'cat','-3' ); // 3 is the category id
	
	return $query;
}


//To exclude custom post type category
add_filter( 'pre_get_posts', 'cp_filter_search' );

function cp_filter_search( $query ) {
	if ( $query->is_search && !is_admin() ) {
    //Define the tax_query
    $taxquery = array(
      array(
        'taxonomy' => 'dt_portfolio_category', // also try lower case, remember a taxonomy name must be in lowercase
        'field' => 'slug',
        'terms' => array( 'projects-2' ), //slug of the portfolio category
        'operator' => 'NOT IN'
      )
    );
      
	  $query->set('tax_query', $taxquery );
   
  }
}

//limit search to a specific post type
function searchfilter($query) {

    if ($query->is_search && !is_admin() ) {
        $query->set('post_type',array('dt_portfolio'));
    }

return $query;
}

add_filter('pre_get_posts','searchfilter');

Add thei snippet in theme's/child theme's functions.php