RN-RNav-9-ResetOnLogin.js

...
import {NavigationActions} from "react-navigation";

class LoginScreen extends React.Component {
  render() {
    const reset = NavigationActions.reset({
      index: 0,
      actions: [NavigationActions.navigate({ routeName: "Internal" })]
    });
    return (
      ...
        <Button
          onPress={() => this.props.navigation.dispatch(reset)}
          title="Login"
        />
      ...
    );
  }
}

LoginScreen.navigationOptions = {
  title: "Login",
  header: null //header is null to hide header
};

...