Ban a user and end their session

I’m working on a plugin derived from this tutorial. So far, the plugin adds a checkbox on a user’s profile edit page where admins can ban the user.

If checked, the user will receive a message upon logging in:

It works fine. The problem is, if the user is already logged in (has an active session) and gets banned, the user can continue to interact with the site until either the session ends or the user logs out.

How do I end the user’s session upon banning so the user is forced out?

Here is the code:

/**
 * Admin init
 *
 * @access      public
 * @since       1.0 
 * @return      void
*/
function rc_admin_init(){

    // Edit user profile
    add_action( 'edit_user_profile', 'rc_edit_user_profile' );
    add_action( 'edit_user_profile_update', 'rc_edit_user_profile_update' );

}
add_action('admin_init', 'rc_admin_init' );


/**
 * Adds custom checkbox to user edition page
 *
 * @access      public
 * @since       1.0 
 * @return      void
*/
function rc_edit_user_profile() {
    if ( !current_user_can( 'edit_users' ) ) {
        return;
    }

    global $user_id;

    // User cannot disable itself
    $current_user = wp_get_current_user();
    $current_user_id = $current_user->ID;
    if ( $current_user_id == $user_id ) {
        return;
    }
    ?>
    <h3>Ban user</h3>
    <table class="form-table">
    <tr>
        <th scope="row"></th>
        <td><label for="rc_ban"><input name="rc_ban" type="checkbox" id="rc_ban"  /> Check to ban user.</label></td>
    </tr>
    </table>
    <?php
}


/**
 * Save custom checkbox
 *
 * @access      public
 * @since       1.0 
 * @return      void
*/
function rc_edit_user_profile_update() {

    if ( !current_user_can( 'edit_users' ) ) {
        return;
    }

    global $user_id;

    // User cannot disable itself
    $current_user    = wp_get_current_user();
    $current_user_id = $current_user->ID;
    if ( $current_user_id == $user_id ) {
        return;
    }

    // Lock
    if( isset( $_POST['rc_ban'] ) && $_POST['rc_ban'] = 'on' ) {
        rc_ban_user( $user_id );
    } else { // Unlock
        rc_unban_user( $user_id );
    }

}


/**
 * Ban user
 *
 * @access      public
 * @since       1.0 
 * @return      void
*/
function rc_ban_user( $user_id ) {

    $old_status = rc_is_user_banned( $user_id );

    // Update status
    if ( !$old_status ) {
        update_user_option( $user_id, 'rc_banned', true, false );
    }
}


/**
 * Un-ban user
 *
 * @access      public
 * @since       1.0 
 * @return      void
*/
function rc_unban_user( $user_id ) {

    $old_status = rc_is_user_banned( $user_id );

    // Update status
    if ( $old_status ) {
        update_user_option( $user_id, 'rc_banned', false, false );
    }
}


/**
 * Checks if a user is already banned
 *
 * @access      public
 * @since       1.0 
 * @return      void
*/
function rc_is_user_banned( $user_id ) {
    return get_user_option( 'rc_banned', $user_id, false );
}


/**
 * Check if user is locked while login process
 *
 * @access      public
 * @since       1.0 
 * @return      void
*/
function rc_authenticate_user( $user ) {

    if ( is_wp_error( $user ) ) {
        return $user;
    }

    // Return error if user account is banned
    $banned = get_user_option( 'rc_banned', $user->ID, false );
    if ( $banned ) {
        return new WP_Error( 'rc_banned', __('<strong>ERROR</strong>: This user account is disabled.', 'rc') );
    }

    return $user;
}

add_filter( 'wp_authenticate_user', 'rc_authenticate_user', 1 );

So the ending of the session should go in the rc_ban_user() function.

EDIT: Full plugin posted below.

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;
}