The way it works, if there are no site-wide user accounts created, the site-wide user login area on the front page will not appear. The exception to this is if you're logged in as site administrator; you'll see the login area, even if your users won't.
What may be showing up for your users is a message and link saying "New user? Register" which prompts them to create their own site-wide user account. To turn that off, go into the main site administration page, and deselect the checkbox labelled "Users can register themselves."
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