prevent submit in ajax call
$form_state variable holds 'triggering_element' element which indicates what button triggered (or also you can use #clicked_button element), example (in ajax callback function):

function my_form_submit($form, &amp;$form_state) {
  //dpm($form_state);
  if($form_state['triggering_element']['#value'] == 'MY_ELEMENT\'S_NAME') {
    //do_operation();
  }
}