Can't disable user registration

On my site http://www.musician.com/harpazio I am getting a ton of obviously spam registrants. How could I disable registration? I looked it up and it says to go to settings "general" but there is no option there. Also, if and when that is disabled can people still sign up through free memberships or courses rather than through http://www.musician.com/harpazio/register . I'd prefer registration through a free course.