For Posterity, and to close up one of the thousands of abandoned Zen Cart posts (I arrived here attempting the same from Google):
There are any number of reasons why you wouldn't want users to register. Google Checkout Mod for instance registers people for you...if you're only offering GC then why would you want to allow registration? Likewise, if you're registering people yourself as the OP was...
Anyway, OP was off in their method, to prohibit the Registration option (tpl_modules_create_accout.php) from showing at all,
1. make a backup of your tpl_login_default.php
2. delete the follwing from tpl_login_default.php:
3. re-up the file.
<?php echo zen_draw_form('create_account', zen_href_link(FILENAME_CREATE_ACCOUNT, '', 'SSL'), 'post', 'onsubmit="return check_form(create_account);"') . zen_draw_hidden_field('action', 'process') . zen_draw_hidden_field('email_pref_html', 'email_format'); ?>
<legend><?php echo HEADING_NEW_CUSTOMER; ?></legend>
<div class="information"><?php echo TEXT_NEW_CUSTOMER_INTRODUCTION; ?></div>
<?php require($template->get_template_dir('tpl_modules_create_account.php',DIR_WS_TEMPLATE, $current_page_base,'templates'). '/tpl_modules_create_account.php'); ?>
<div class="buttonRow forward"><?php echo zen_image_submit(BUTTON_IMAGE_SUBMIT, BUTTON_SUBMIT_ALT); ?></div>
this way you're removing the entire call to FILENAME_CREATE_ACCOUNT, the 'box' and all.
Easy Peasy. Tested and working on 1.3.9h