What I would do is use the element inspector to go through the divs inside topribbon, and one by one adding display: none to each until you lose the scroll bar.
i had a quick look and found that the div with id ‘ads’, which is the containing div for your advert has a width of 1010px, the same as the width of your content, but as the div starts 1/2 way along the page, it causes the scroll bar to appear.
I would look into setting the div width for ‘ads’ to the same as the width of the advert inside it.
With firebug, if you click on the element inspector (looks like a blue cursor clicking a box), then hover just to the right of the advert, you should see the div container heading off the edge of the screen to the right