Make the subject box hidden in the reply form.

Change this:

<input type="text" name="Subject" maxlength="{$config['subjectlength']}" class="formboxes" size="60" value="$Subject" /><br />


To:


<input type="hidden" name="Subject" maxlength="{$config['subjectlength']}" class="formboxes" size="60" value="$Subject" /><br />


This way they cannot change it.


--
Website Development and Management
www.jcswebdev.com