Well, if anyone really does feel the need to get rid of the ability to change the topic subject in a reply you would pop open your copy of templates/default/newreply.tpl and...

find:
PHP Code
{* NO SUBJECT FOR GALLERY REPLIES *}
{if !$is_gallery}
<tr>
<td class="alt-2">
<label for="Subject">{$lang.SUBJECT_TEXT}</label>
</td>
<td class="alt-1" valign="top">
<input type="text" name="Subject" id="Subject" maxlength="{$config.SUBJECT_LENGTH}" class="form-input" size="60" tabindex="1" value="{$Subject}" />
</td>
</tr>
{$iconselect}
{/if}

and replace it with:
PHP Code
{* Begin Total Annihilation of Subject *}
{if $annihilated}
{* NO SUBJECT FOR GALLERY REPLIES *}
{if !$is_gallery}
<tr>
<td class="alt-2">
<label for="Subject">{$lang.SUBJECT_TEXT}</label>
</td>
<td class="alt-1" valign="top">
<input type="text" name="Subject" id="Subject" maxlength="{$config.SUBJECT_LENGTH}" class="form-input" size="60" tabindex="1" value="{$Subject}" />
</td>
</tr>
{$iconselect}
{/if}
{/if}
<input type="hidden" name="Subject" value="{$Subject}" />
{* End Total Annihiliation of Subject *}

then rip into templates/default/editpost.tpl...

find:
PHP Code
<tr>
<td class="alt-2">
<label for="Subject">{$lang.SUBJECT_TEXT}</label>
</td>
<td class="alt-1">
<input type="text" id="Subject" name="Subject" value="{$Subject}" maxlength="{$config.SUBJECT_LENGTH}" class="form-input" size="60" />
</td>
</tr>

and replace with:
PHP Code
{* Begin Total Annihilation of Subject *}
{if $annihilated}
<tr>
<td class="alt-2">
<label for="Subject">{$lang.SUBJECT_TEXT}</label>
</td>
<td class="alt-1">
<input type="text" id="Subject" name="Subject" value="{$Subject}" maxlength="{$config.SUBJECT_LENGTH}" class="form-input" size="60" />
</td>
</tr>
{/if}
<input type="hidden" name="Subject" value="{$Subject}" />
{* End Total Annihilation of Subject *}