To access the session object in your action class, it should implement the interface : org.apache.struts2.interceptor.SessionAware.
Refer to this for more details on the SessionAware interface. Once you have access to the session object, add the below piece of code to invalidate the session (typically used for logout mechanism in webapps).
if (session instanceof org.apache.struts2.dispatcher.SessionMap) {
try {
((org.apache.struts2.dispatcher.SessionMap) session).invalidate();
} catch (IllegalStateException e) {
logger.error(msg, e);
}
}