The Sticky Multiple Account Information add-on detects, reports and permanently stores information about multiple accounts. It is not an automated user management or user disciplining system. It helps you and your moderators to deal with people using more than one account at your forum.
It can optionally notify about detected accounts using several notification methods. Since the information about multiple accounts is stored and always provided to be easily accessible for all people whom you give permission to, it is not always necessary to activate notifications.
It's settings optionally allows to automatically deny registration of a new account, if the person registering already has an account. It also optionally allows to set all detected multiple accounts to "moderated" (you need to manually approve them).
Further than that, you need to manually deal with detected multiple accounts.