Blocking a certain user from apearing in user search results
If you're using drupal search out of the box to search users, that should still call implementations of hook_search_page

Build a small custom module that implements hook_search_page, parse thru $results and remove anything you don't want appearing in search results.

function custom_module_search_page($results) {
  foreach($results as $result_index =&gt; $result) { 
    //do something, maybe detect for a user id
    //and run unset($results[$result_index]); if the criteria is met
  }
}