Hi,
1. You can turn off session management in transaction SICF_SESSIONS. I would not do this.
2. You need to let portal know that backend is using new session management. In system landscape, go to backend properties and look for attribute with *ABAP*. You just need to tick this box and portal will start calling logoff service that kills session on backend system. The only issue I can see is if your version of portal does not supports this.In this case you will have to go with option 1.
There is also OSS note for this. So just search for HTTP session or something like that.
Cheers