Find 70% in that template and change it to a fixed width. If your forums are 800 wide I would say make it 640 or 760.
Code:
<table class="tborder" cellpadding="$stylevar[cellpadding]" cellspacing="$stylevar[cellspacing]" border="0" width="70%" align="center">
Code:
<table class="tborder" cellpadding="$stylevar[cellpadding]" cellspacing="$stylevar[cellspacing]" border="0" width="760" align="center">
You'll also have to do this for the REGISTER template, and others. I had to when changing to fixed width, and I'm not quite sure why vBulletin didn't set it up so that it always followed the width you had set in the style.