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In admin>preferences>user info settings turn 'allow new user registrations to 'no' this will stop people from registering on your site.
If your theme has a logon(user/pass)box on it edit theme.html to remove the login form. Go to blocks(in admin) and make sure the login block is turned to visible 'no'
You could also either redirect user.php and register.php back to your main site or maybe remove those files completely.
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