Concatenating the all above written:
1) set
$enableNewRegistrations (setup_options.php) to FALSE;
2) Edit templates/main_header.html and remove {$l_menu[2]} ;
3) Possibly edit templates/user_login_form.h
tml and eliminate the Password field (so users don't need to enter the password, but if they will be required to enter a nickname, pay attention at Required Nickname solution from Downloads;
4) Don't forget to purchase the Captcha addon ;-) It will protect your board from anonymous automated spam and flood...