How do I remove a validation handler from a form? I am using the Invite by e-mail module, which has its own hook_form_alter() with a validation handler. I wish to remove its validation handler and use my own validation handler.
Use hook_form_alter() to change the $form['#validate'] to be your custom validate handler.

function custom_module_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if ($form_id == 'your_form_id') {
   $form['#validate'] = array();
   $form['#validate'][] = 'my_custom_validation'; 
  }
}