WTF!? You don't know how to hide a module? The login form (mod_login) is nothing but another built-in module. Just like the Main Menu (mod_mainmenu), the polls (mod_poll), so on and so on. All modules are managed form the Module Manager:
Click Sequence #1: Modules -> Site Modules
hide login form
Now search for the "Login Form" and unpublish it. You can do a "Select type' mod_login, or type "Login" in the "Filter" field. BAM! No more login form on the front-end. Who would want to do such a thing? Many people, said Mr. Zen. Many people....