Disabling sidebar for a particular page

Hi there. Here’s a noob question.

My current site (https://www.tanggam.com) have a sidebar on the right. I created this using artisteer over a year ago. Now I want to have a page that does not have a sidebar and basically have the content part stretched to the width of the wrapper.

I managed to figure out the template thingy and currently using a template called Tanggamforum2.php, deleted the sidebar line in that file and selected the template in the page editor. But I can’t make the content width stretched to the wrapper. This is what it looks now: https://www.tanggam.com/v4/thesis-2004-semester-2/

Kindly assist what I’ve been missing. I’m not a programmer or anything, so excuse my shortcomings ;).