If you have a course group link in your online registration site's left nav that you'd like to hide from students who have not yet satisfied the prerequisite for taking a class from that course group, it's easy to do.
Just go to Configure > Preferences, and then at the bottom of the third green box on the page you'll see a preference named "Hide Courses/Groups with Prerequisites." Click the Edit button, then click the checkbox for that preference, then click the Save button.
All course groups links that have prerequisites will now be hidden from your left nav, unless the student has satisfied those requirements. The links will also be hidden to anyone who is not logged into the system, as a student must be logged in in order for us to verify that they have completed the requirements).