How to perform this in CodeIgniter MVC

That's where the model concept comes in.
Create a model like this:
Filename: application/models/User_model.php

PHP Code:

class User_model extends CI_Model {
  public function get_names_without_aaa()
  {
     $users = array();
     $query = $this->db->get('users');    //get is a function in the query builder!
     foreach ($query->result_array() AS $row) {
        $FirstName = str_replace('aaa','***',$row['FirstName']);
        $users[] = array("FirstName"=>$FirstName);
     }
     return $users;   
  }
} 


In the controller:

PHP Code:

$this->load->model('user_model');
$data['users'] = $this->user_model->get_names_without_aaa();
$this->load->view('myview',$data); 


Now, you can load the model in any controller you want, or even autoload it in application/config/autoload.php.