Firebase Simple Login is a service built on-top of Firebase Custom Login, and provides useful primitives for authenticating users via common means.
By design, Firebase Simple Login does not give you access to the users' passwords programmatically, as it only increases the risk that they are not handled or stored securely. Today, the only two methods that can be used to remove an email / password hash mapping is either via the client API using the email and password, or via the admin panel at https://<YOUR-FIREBASE>.firebaseio.com
.
Keep in mind that when using email / password login, Firebase Simple Login simply creates a new mapping between an email address and a password hash, but does not store any information in your Firebase. Also note that there is no way to "blacklist" a user id, so if you remove the mapping, the user could re-create it.
If you want to ban / block users, a better approach would be to create a new list in Firebase of your "blacklisted" users, and then use security rules to ensure that that user is in the list (i.e. user is blocked if root.child('blocked-users').hasChild(auth.uid)
).