thomgann 
I think you can comment out following code, but not sure if that will break anything else
/include/staff/ticket-open.inc.php
Lines 500 to 510 comment out
/*
 <?php
    // Popup user lookup on the initial page load (not post) if we don't have a
    // user selected
    if (!$_POST && !$user) {?>
    setTimeout(function() {
      $.userLookup('ajax.php/users/lookup/form', function (user) {
        window.location.href = window.location.href+'&uid='+user.id;
      });
    }, 100);
    <?php
    } ?>  */
If you want to see the dialog again, just click on the "Add New" it will pop up
