If you are using vbindex just change the following to prevent showing the preview on mouse over threadtitle of lastest threads for unregistered people.
find in Template
vbindex_latestthreadbit :
PHP Code:
<a href="$vboptions[bburl]/showthread$ext?$session[sessionurl]threadid=$thread[threadid]" title="$thread[preview]">
and replace it with:
PHP Code:
<a href="$vboptions[bburl]/showthread$ext?$session[sessionurl]threadid=$thread[threadid]" <if condition="$show['member']">title="$thread[preview]"</if>>