You should check the documentation on CAS single sign out which can be found here. It says:
Where Single Sign Out Works:
Clients / Languages whose session management is maintained on the
server side. CAS clients can then access session information to end
the session.
Where Single Sign Out Doesn't Work:
Clients / Languages whose only session management consists of cookies.
CAS does not have access to the browser's cookies and cannot therefore
terminate a session. (however, closing a browser window should do
that)
And I think that's pretty much your case. Cookie based it's not possible to do a SSout, but if you can use a framework like Spring (as we do in our projects) the SSout is easy to configure and comes in very handy