You ned to understand the terminology that WordPress uses (this is NOT theme related, it applies to every theme) and it can get a little confusing.
In the backend, WordPress has posts and pages. A page is actually a special type of post.
A post is an entry that makes up your blog, post tend to happen frequently.
A page is comprised of static data.
In the front end, what you see on the screen is considered a page. Your blog page generally is composed of many posts. This is referred to as a 'multi-post' page. When you look at a single post, this is referred to as a 'single post' page. When you look at a actual page this is referred to as a 'page' page.
WordPress also uses the terminology homepage and front page. The home page references your blog page (why they didn't call it the 'blog' page I have no idea). The front page is the page the user will see when they type in your URL. The page that shows when you first enter your URL is defined in the reading settings –.front page displays.
If you have set 'front page displays' to 'your latest posts' - then the page you see when you type in your URL is called both the 'front page' and the 'home page'.
If you have set 'front page displays' a static page, then the front page would select will be your 'front page' and the 'post page' will be called the 'home page'.
(are you confused yet? It took me a while to get this straight in my own mind.)
A category page is a list of posts from a particular category.
So now to your problem. If you do not want the left sidebar to show on all category pages, simply go to the 'style and configure sidebars 'section and in the 'left sidebar: display on:' uncheck the 'category pages' option.
that should be all you need to do.