@User , so just to clarify, you can disable the sign up API as follows:

EmailPassword.init({
    override: {
        apis: (oI) => {
            return {
                ...oI,
                signUpPOST: undefined
            }
        }
    }
})

Finally, you can manually sign up users like this:

let signUpResponse = await EmailPassword.signUp("email", "password");

if (signUpResponse.status === "OK") {
    let user = signUpResponse.user;
    // TODO:...
} else if (signUpResponse.status === "EMAIL_ALREADY_EXISTS_ERROR") {
    // TODO: handle this error
}

And as @User said, the above works because

EmailPassword

is a singleton. Thank you @User