Ban a user and end their session

Use wp_logout(). It calls wp_clear_auth_cookie() and invalidates the current log-in information immediately.

Sample code, not tested:

add_action( 'init', 'log_out_banned_user' );

function log_out_banned_user() {
    if ( ! is_user_logged_in() )
        return;

    $user = wp_get_current_user();

    if ( ! get_user_option( 'rc_banned', $user->ID, false ) )
        return;

    wp_logout();
    wp_redirect( home_url( "https://wordpress.stackexchange.com/" ) );
    exit;
}