This appears to be related to your prior post:
http://www.zen-cart.com/forum/showthread.php?t=50465
A number of steps are involved, including:
a. in the tpl_login_default.php, remove the call to the create_account module. (the section related to New Customers)
b. rename the /includes/modules/pages/create_account folder to create_account_OFF
this will cause page_not_found to trigger if someone mistakenly attempts to access the create_account page
c. You may have to alter the zen_customer_greeting() function if you plan to use the greeting feature, so that it doesn't offer a link to the create_account page. However, it's likely easier to simply remove the "create an account" text from the TEXT_GREETING_GUEST definition in english.php
d. the on_load_main.js file in the includes/modules/pages/login will have to be adjusted to not check the create_account form contents