Login Error in Webtop 5.3

We have customized login component in Webtop 5.3 SP2 and have overwritten the onLogin method.
The requirement is to prevent the users belonging to group "test_users" to be able to login. We display an error message and prevent then from logging in.

In the onLogin() method, I have included the below code:

IDfGroup urlUsersRoleGroup = getDfSession().getGroup("test_users");
if(urlUsersRoleGroup != null) {
isURLUser = urlUsersRoleGroup.isUserInGroup(m_strUsername);
}
if(!isURLUser) {
handleSuccess(m_strDocbase, m_strUsername, strPassword, m_strDomain);
}
else {
throw new Exception(this.getString("ERR_URL_USER"));
}

Finally, I disconnect the session in the finally block.

The issue is:

When a user in the group "test_users" tries to login, it displays the error message on the login screen and doesn't log him in.
Now if a user who doesn't belong to that group logs in, it gets the previous session and takes him to the inbox of the user in the group "test_users" whom I tried to login previously. I am not able to get rid of the previous session even if I disconnect the session.

Any help would be highly appreciated.