How can I prevent a form from being displayed in the privatemsg module?
The following code does it for me:

function MYMODULE_form_pm_block_user_list_alter(&amp;$form, &amp;$form_state, $form_id) {
  unset($form['new']);
}


This should help if you didn't know about it.  

hook_form_FORM_ID_alter();

Also there's the more generic hook_form_alter().