Really? I certainly had no luck doing so.
Open a support ticket and send them this:
Please remove my Subscription to _______ game pursuant to the Steam Subscription Agreement.
13. TERM AND TERMINATION
Either you or Valve has the right to terminate or cancel your Account or a particular Subscription at any time. You understand and agree that the cancellation of your Account or a particular Subscription is your sole right and remedy with respect to any dispute with Valve.
I understand that I will need to acquire a new activation code and/or repurchase the game if I want to have it back in my Steam Library.