Close a wordpress blog – keep site as it is but prevent hacks

Why not just disable comments and registration?

This comes to mind also:

(Redirect all requests to login page or admin pages to homepage. A little irreversible.)

$currentURL = $_SERVER["HTTP_HOST"] . $_SERVER["REQUEST_URI"];
    if (strpos($currentURL, 'wp-admin' ) or  strpos($currentURL, 'wp-login' )) {
                    header( 'Location: '.site_url() );
}

Caution: this stops you from logging in also.

Edit:

And adaptation of the above code put into plugin format can be found here. Thanks to brasofilo.

Modified code:

add_action( 'init', function()
{
    $currentURL = $_SERVER["HTTP_HOST"] . $_SERVER["REQUEST_URI"];
    if( strpos( $currentURL, 'wp-admin' ) or strpos( $currentURL, 'wp-login' ) )
        exit( wp_redirect( site_url() ) );
} );