Currently, state is released by setting the value for the key to null. If you are tracking web sessions, you can try and send a "end of session" element that sets the value to null.
To be on the safe side, you probably want state that is automatically purged after a while. I would look into using Windows for that. The triggers there are flexible so you can schedule both actions on elements plus cleanup after a certain time delay (clock time or event time).
The question about "state expiry" has come a few times. People seem to like working on state directly, but it should clean up automatically.
Can you see if your use case fits onto windows, otherwise open a ticket for state expiry?