Problem: You want to be able to login to the front end, but you don't want a login form on your site.

Solution: Type this into the browsers address bar. Put your domain name in of course. 

http://mysitename.com/index.php?option=com_users&view=login 
or
http://mysitename.com/component/users/?view=login

Keep in mind that all Joomla sites have this feature. You are not stopping people, including hackers, from being able to login to the front end of your site by un-publishing the login module.

If logging in to the front end is a common necessity for your site, but you don't really want the login module displayed on your pages, you can add a login page as a separate Menu Item outside your regular navigation menus.

You may have other contributors and article editors that regularly need to get to the front end, but again, you don't want the login module to appear on the regular site.

If this is the case, read this joom3 article: How to create a separate login page for my Joomla website.