[blind-eddie]

I don't believe there is a way to disable just that section.....
You could add if conditionals to the find all post by user link in the postbit/postbit legacy (Which ever you use) and the MEMBERINFO templates to hide the link from all but admins.
But, if someone knows the URL they will still be able to see post by user.

Example:
In your postbit_legacy template the code for the find more post by user link is...

	<if condition="$show['search']">
		<tr><td class="vbmenu_option"><a href="search.php?$session[sessionurl]do=finduser&amp;u=$post[userid]" rel="nofollow"><phrase 1="$post[username]">$vbphrase[find_more_posts_by_x]</phrase></a></td></tr>
	</if>

To hide that link from all but admins you will be using the following conditional.

<if condition="$post['usergroupid'] == 6"></if>

After adding conditional:

               <if condition="$post['usergroupid'] == 6">
	<if condition="$show['search']">
		<tr><td class="vbmenu_option"><a href="search.php?$session[sessionurl]do=finduser&amp;u=$post[userid]" rel="nofollow"><phrase 1="$post[username]">$vbphrase[find_more_posts_by_x]</phrase></a></td></tr>
	</if>
               </if>

To learn more about using vBulletin Template Conditionals please visit this Article.

Happy editing.