WordPress - disable admin bar.

functions.php

/**
 * Hide admin bar for all.
 */
add_filter('show_admin_bar', '__return_false');

or

/**
 * Show admin bar only to those who can edit posts.
 */
function dhf_hide_admin_bar() {
  if (!current_user_can('edit_posts')) {
    show_admin_bar(false);
  }
}
add_action('set_current_user', 'dhf_hide_admin_bar');