Sorry to revive such an old subject. I've been fiddling around with this a bit and I haven't been able to find a good work around, only a half hearted one. In Site administration > Appearance > Navigation there is an option called navcourselimit. I have set that to 0. It doesn't seem to have an effect on the courses shown in the front page, however. I wonder what it affects?
In Site Administration > Front page > Front page settings I've set the frontpage course limit to two. That means that people can only see two people on their front page at a time, even if they are enrolled in a course: but as our students rarely enroll in more than one course at a time, two should be enough and that means the damage is at least limited to two course visible for the user who has an account but is not enrolled in a course... Maybe this work-around works for someone else as well.
Now if I could just get rid of that 'All courses' link, my life would be a lot easier. I'll try your workaround Thorsten, as soon as I have an ok from my fellow administrator who knows a lot more about css than I do Thanks so much for that! If it works I'm going to start my weekend in a very good way.