Add validation on views bulk operations form
Your omhm_mod_views_bulk_operations_form_alter($form, &amp;$form_state, $form_id) takes $form by value, not by reference, so any changes made to $form are scoped to the function only. That's why $form['select']['submit']['#validate'][] changes nothing. It should be omhm_mod_views_bulk_operations_form_alter(&amp;$form, &amp;$form_state, $form_id) 

When you fix mistake I pointed above, and it will not help, be sure to test if your hook is run at all, and if it is, then test if your validator is. It'll be needed for your next question, if you will need to ask it.