Disable Email Login for WP-Admin

functions.php

<?php
remove_filter( 'authenticate', 'wp_authenticate_email_password', 20 );

/**
 * Disable logging in via email
 *
 * Check for and serve an appropriate response to users attempting to sign in with email.
 *
 * @param  string $user     If the user is authenticated.
 * @param  string $username Username or email address.
 * @return string           Authentication details
 */
function disable_email_login( $user, $username ) {

	if ( ! empty( $username ) && filter_var( $username, FILTER_VALIDATE_EMAIL ) ) {
		return new WP_Error( 'email_login_disabled', __( 'Logging in with email is disabled.', 'text-domain' ) );
	}

	return $user;
}

add_filter( 'authenticate', 'disable_email_login', 20, 3 );
?>

Script created by my colleague @f0rthelulz, for a WordPress VIP customer.

Can be added to functions.php and will then prevent an email address from being used to login - usernames only can be used. Just change text-domain to, well, your text domain for the error translation to work.