You can't prevent users from being created on the client using simple login. There are two options you can utilize instead:
Simple Login "accounts" are really just tokens
Simple Login is just a convenience wrapper that creates Firebase tokens. There is no limit on how many accounts can be stored and they have no affect on your Firebase usage. With this in mind, there's really no reason you need to restrict creation of accounts.
Instead, just utilize security rules to control access to data. When an admin creates an account, have them also add a profile into the data. If only an admin in Forge is allowed to create the profile, then someone could create an account, but it would be superfluous and pointless, since all it does is give them an inert token.
A security rule to enforce access to data:
".write": "root.child('valid_account/'+auth.uid).exists()"
A security rule that allows users to edit their profile but only Forge (admin: true) to create them:
"profiles": {
"$uid": {
".write": "data.exists() && auth.uid === $uid && newData.exists()"
}
}
Creating your own tokens allows complete control
If you're terribly OCD and don't like that approach, then you can cut out Simple Login. As stated previously, it just creates tokens on your behalf. So simply create your own.
In this way you have complete control over account creation and token generation.