After one-click unsubscribe web site may email the user last notification that unsubscribe was successful, and if the user wants to subscribe again - he can do so by clicking on that re-subscribe link (which can also be one-click).

Update: I just noticed that this goes against Amanda Peyton's last request (Do not send an unsubscribe confirmation email.). But I guess she's wrong on that (because unsubscribe could be unintentional or malicious based on forwarded email).