It is possible to disable the signUp component: You want to do the below on the frontend

EmailPassword.init({
  signInAndUpFeature: {
    signInForm: {
      style: {
        headerSubtitle: {
          display: "none", // this will hide the sign up button in the sign in form
        }
      }
    }
  },
  override: {
    components: {
      EmailPasswordSignUp: (_) => (_) => {
        return null; 
      }
    }
  }
})

You want to the below on the backend:

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