Guido, when you are at the login page, there is a gray bar across the top, above where you would enter your username. That is what you click to get to the course database.
When you block a person, they can no longer invite you to a private message or post to your profile wall. Replies and comments they make will be collapsed/hidden by default. Finally, you'll never receive email notifications about content they create or likes they designate for your content.
Note: if you proceed, you will no longer be following .