Unfortunately there isn't a easy way to edit the html in the control without the source.
The best way to edit the html I can think of is to make a xmlpackage/control that uses that uses a forum outside the runat sever tag. Then have have action set to a new page that processes the forum data. Depending on if you want to use captcha or not it may get a bit more complex but the basic html without captcha would look something like this.
C#/VB.NET Code:
<form action="Newsletter_Sub.aspx" method="post" enctype="multipart/form-data">
Email Address: <input type='text' id='txtEmailAddress' size='13' /><input type='submit' value='Submit'id='cmdSubmit' />
</form>