Show subscription requests only when a contact is in the contact list already

subscription authorization was a good way to protect against spammers. Until the amount of subscription requests was small enough to be handled manually. Now it's way too many. Everyone who pressed the "unsubscibe" button for dozens spammers after each registration should understand what I mean.

Is it possible to add new configuration option: "Show subscription requests only when a contact is in the contact list already"?

If such option is disabled then let's things go as usually. But if the option is enabled then I'd like to see requests only from people from my roster. Believe me, my friends have enough ways to give me their contact info.