option in user cp to mass unsubscribe from all old threads

[jilly]

I get this request a lot - we need an option by the subscribed threads link in the user cp to allow users to 'unsubscribe from ALL threads' with one click.

They can turn off email notification in their edit options, which will stop all NEW email notifications of newly subscribed threads or whatever, but that doesn't affect their previous subscriptions, and they need a way to mass unsubscribe from old threads without having to individually go in and unsubscribe from each old thread.