Force Users To Relogin

The wp_logout function will log a user out. Here’s an example that will log someone out and redirect to the login page when the url contains the query string ?my_logout_flag:

add_action('init', 'wpse52743_check_logout');

function wpse52743_check_logout(){
    if( isset( $_GET['my_logout_flag'] ) ) {
        wp_logout();
        wp_redirect( wp_login_url() );
    }
}