If you want to remove the "log in as guest" button then go to site administration on the
front page>users>authentication>manage authentication and scroll down to where it says "Guest login button -" and choose the hide option. If you set your front page (front page settings) so that courses are only visible once people have logged in then the guests won't even see the courses beforehand. (However, it is possible for anyone to see the list of courses if they know the
URL to get to them so even though the guest login button is not visible or you have elected not to display your course list, if anyone was determined enough they could see your course list -but I wouldn't worry about it!)