Force a redirect if a certain username is attempted.

disable-username-login.php

<?php

add_filter( 'authenticate', 'rkv_disable_username_logins', 20, 3 );
/**
 * Disable admin username and redirect to Google (or another site).
 *
 * @param  object $user      The WP_User object.
 * @param  string $username  The entered username.
 * @param  string $password  The entered password.
 *
 * @return object $user      The WP_User object, or redirected.
 */
function rkv_disable_username_logins( $user, $username, $password ) {

	// If we have no username to check, just bail right away.
	if ( empty( $username ) ) {
		return $user;
	}

	// Set our array of usernames to check for.
	$names  = apply_filters( 'rkv_disabled_username_list', array( 'admin' ) );

	// Bail if the name is empty.
	if ( empty( $names ) ) {
		return $user;
	}

	// Cast our names list to be an array (in case someone messed up the filter).
	$names  = (array) $names;

	// Set our array of usernames to check for.
	$site   = apply_filters( 'rkv_disabled_username_site', 'https://www.google.com' );

	// Only run this on the names set.
	if ( in_array( $username, $names ) ) {
		
		// Double check we have a site to use.
		$site   = ! empty( $site ) ? $site : 'https://www.google.com';
		
		// Redirect to our chosen site.
		wp_redirect( esc_url( $site ) );
		exit;
	}

	// Return the user.
	return $user;
}