You would need to edit the template and wrap it in a conditional.
How can I show content just to logged in members and hide it from guests?
<xen:if is="{$visitor.user_id}">
This content will show to logged in members only
</xen:if>
How can I find out which template to edit?
Using your browser, view the page source and look for a line of code near the top of the page which begins <div id="content" class="; the class is the name of the template. So for the main forum page, the line of code is: <div id="content" class="forum_list">, which makes the template forum_list.