How do I disable a lightbox form for known visitors

I understand how to add custom text for know visitors on lightbox popup form, but I don't want it to show anything for known visitors. 

I thought that I could do something like add this into the custom text:

<script language="javascript" type="text/javascript">

MktoForms2.lightbox(form).hide();

</script>

But the known text doesn't seem to like JavaScript.

I can't be the only one that needs this functionality and I can't find anything in the documentation.  Is this possible?