I typically use ssh with connection sharing enabled; this causes any ssh tunnels I create subsequent to the initial ssh login to use that connection. Is it possible to close an ssh tunnel without killing the ssh process in such circumstances?
You can terminate all the shared sessions by issuing
-O exit from the client side.
ssh -O exit masterhostname
Sadly there is no way to terminate a specific session and leave the others running.
Sessions usually terminate naturally when the session is closed (EOF). Tunnels made using -L and -R are always listening for new sessions and there is no way to indicate they should stop listening.