hello

open:
themes/default/subtemplates/subnavigation_2.inc.tpl

find:

{elseif $mode=='thread'}
<ul id="subnavmenu"><li>{if $usersettings.thread_display==0}<a class="linear" href="index.php?mode=thread&amp;id={$id}&amp;toggle_thread_display=true" title="{#thread_linear_linktitle#}">{#thread_linear#}</a>{else}<a class="hierarchic" href="index.php?mode=thread&amp;id={$id}&amp;toggle_thread_display=true" title="{#thread_hierarchical_linktitle#}">{#thread_hierarchical#}</a>{/if}</li>

and delete it.

good luck