Filters out matches outside custom fields

relevanssi-customfield-filter.php

<?php

add_filter( 'relevanssi_match', 'rlv_customfield_match_filter', 10 );
function rlv_customfield_match_filter( $match ) {
  global $wp_query;
  if ( 'listing' === $wp_query->query_vars['post_type'] ) {
    // This is a listing search.
    if ( $match->customfield < 1 ) {
      // No custom fields were hit, set weight to zero.
      $match->weight = 0;
    }
  }
  return $match;
}