Edit /templates/default/subtemplates/index.tpl.inc

<strong> {if $data.$element.user_type==2}<span class="admin" title="{#administrator_title#}">{$data.$element.name}</span>{elseif $data.$element.user_type==1}<span class="mod" title="{#moderator_title#}">{$data.$element.name}</span> {else}{$data.$element.name}{/if} </strong>

Remove the <strong> and </strong>. You've inspired me to do this to my forum too :)

note: I couldn't post this because one of the lines was too long & had to add some spaces, so it won't exactly match the template.