Restricting the WordPress admin-area to a specified user

wp-restrict-admin-to-user.php

<?php
add_action( 'init', 'ejo_allow_user_only' );

/**
 * Restrict Admin section to specified username
 *
 * Prevent people from using the admin-section.
 * Only allow user with username `REPLACE_THIS`
 *
 * WordPress 4.7.0+
 */
function ejo_allow_user_only() {
	
	// Skip if no admin section or if doing an ajax request
    if ( ! is_admin() ) || wp_doing_ajax() ) {
		return;
	}

	// Shut the door for users who don't match username
    $current_user = wp_get_current_user();
	if ( $current_user->user_login != 'REPLACE_THIS' ) {
        wp_redirect( home_url() );
        exit;
    }
}