I assume you’re referring to the black bar that may run across the top of a web page. That’s called the ‘Toolbar’. However, an earlier version of it was called the ‘Admin Bar’, and that name is still used in the code.
To never display it, add this to functions.php
function my_function_admin_bar(){
return false;
}
add_filter( 'show_admin_bar' , 'my_function_admin_bar');
or to just show the bar for admins
function my_function_admin_bar($content) {
return ( current_user_can( 'administrator' ) ) ? $content : false;
}
add_filter( 'show_admin_bar' , 'my_function_admin_bar');
(Or similarly in a plugin.)
See the Codex.
(Also, you should be able to use wp_signon()
if you wish.)