You can put a condition around it:
HTML Code:
<if condition="$forumid != x">show the code for last post<else />(insert non-breaking space or a dash here)</if>
To find the template to modify, do this - vboptions > General Settings > Add Template Name in HTML Comments > set to Yes . Then go back to your page and view the source code and you will see the name of the template called around your part of the code. (Sometimes the template is the one mentioned at the very top of the page source.)