As far as I can tell, there is no option at this time for cancelling a local port forward other than exiting your connection. It is possible to cancel a reverse port forward (from the remote machine back) using this syntax:
<RET><RET>~c (press return twice and ~c to get the ssh command prompt)
This will cancel the port you've forwarded from the remote machine on port 9090 back to your local machine localhost:9090. I tried doing -KL 9090:localhost:9090 and it just said not supported. So I guess eventually it will be possible, but not right now. There is also an option called CancelAllForwardings, but unfortunately you can't use these options either inside the client console.
I tested this with OpenSSH 5.5p1 on Ubuntu, maybe a newer version does have the option, but I didn't see anything about it in the openssh changelog.