This is to make the 'title' field UNEDITABLE in new replies so only the title from the thread is used.
In the 'newreply' template,
Find this code:
<td class="smallfont" colspan="3">$vbphrase[title]: on the same line or below it, add e.g.
<font class="smallfont">(uneditable since replying)</font>
NEXT, scroll a few lines down and...
Find this code:
<input type="text" class="bginput" name="title" value="$title" size="50" maxlength="85" tabindex="1" title="$vbphrase[optional]" /> and