How can I throw a form error if a submission field has a certain value in webform?
Create your own validation function of your webform and do it there:

function modulename_form_webform_client_form_FORMID_alter(&amp;$form, &amp;$form_state, $form_id) {
     //FORMID - your webform id
    $form['actions']['submit']['#validate'][] = 'modulename_form_webform_client_form_FORMID_validate';

}

function modulename_form_webform_client_form_FORMID_validate($form, &amp;$form_state) {
    //validate it here
    if ($form_state['values']['yourvalue'] == 'somevalue') {
        form_set_error('you can\t do that');
    }
}


also you can use hook_form_alter to provide your own validate function.
Further reading form_set_error, hook_form_alter,