[kh99]

I haven't tested this a lot, but you could try this: create a plugin using hook location private_insertpm_process and this code:

if (!is_member_of($vbulletin->userinfo, 6))
{
    $check_recipients = $pmdm->info['recipients'];
    if (!empty($check_recipients))
    {
        foreach ($check_recipients AS $userid => $user)
        {
            if (!is_member_of($user, 6))
            {
   	       eval(standard_error("Private messages allowed only to Admins"));
            }
        }
    }
}