It isn't an option - you would have to modify a template to not show the link to non-logged in users. In your FORUMHOME template you will see a line like this:
<if condition="$vboptions['forumleaders']">
<a href="showgroups.php$session[sessionurl_q]" rel="nofollow">$vbphrase[view_forum_leaders]</a></if>
Modify it to put a condition around it to not show it to guests, like this:
<if condition="!$show['guest']">
<if condition="$vboptions['forumleaders']">
<a href="showgroups.php$session[sessionurl_q]" rel="nofollow">$vbphrase[view_forum_leaders]</a></if>
</if>
This will simply remove the link from the forumhome page. If the user knows the page url, they can still go view it. You would have to modify the permissions on that page to not show it to guests if that is what you need.